Semiconductor device and manufacturing method thereof

ABSTRACT

A minute transistor is provided. A transistor with low parasitic capacitance is provided. A transistor having high frequency characteristics is provided. A semiconductor device including the transistor is provided. A semiconductor device includes an oxide semiconductor, a first conductor, a second conductor, a third conductor, a first insulator, and a second insulator. The first conductor overlaps with the oxide semiconductor with the first insulator positioned therebetween. The second insulator has an opening and a side surface of the second insulator overlaps with a side surface of the first conductor in the opening with the first insulator positioned therebetween. Part of a surface of the second conductor and part of a surface of the third conductor are in contact with the first insulator in the opening. The oxide semiconductor overlaps with the second conductor and the third conductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/073,639, filed Oct. 19, 2020, now allowed, which is a continuation ofU.S. application Ser. No. 16/543,684, filed Aug. 19, 2019, now U.S. Pat.No. 10,879,381, which is a continuation of U.S. application Ser. No.15/822,284, filed Nov. 27, 2017, now U.S. Pat. No. 10,446,671, which isa divisional of U.S. application Ser. No. 15/056,356, filed Feb. 29,2016, now U.S. Pat. No. 9,865,712, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2015-041204 on Mar. 3,2015, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to, for example, a transistor, asemiconductor device, and manufacturing methods thereof. The presentinvention relates to, for example, a display device, a light-emittingdevice, a lighting device, a power storage device, a memory device, aprocessor, and an electronic device. The present invention relates to amethod for manufacturing a display device, a liquid crystal displaydevice, a light-emitting device, a memory device, and an electronicdevice. The present invention relates to a driving method of asemiconductor device, a display device, a liquid crystal display device,a light-emitting device, a memory device, and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A display device, a light-emitting device, a lightingdevice, an electro-optical device, a semiconductor circuit, and anelectronic device include a semiconductor device in some cases.

BACKGROUND ART

In recent years, a transistor including an oxide semiconductor hasattracted attention. An oxide semiconductor can be formed by asputtering method or the like, and thus can be used for a semiconductorof a transistor in a large display device. In addition, the transistorincluding an oxide semiconductor is advantageous in reducing capitalinvestment because part of production equipment for a transistorincluding amorphous silicon can be retrofitted and utilized.

It is known that a transistor including an oxide semiconductor has anextremely low leakage current in an off state. For example, alow-power-consumption CPU utilizing a characteristic of low leakagecurrent of the transistor including an oxide semiconductor is disclosed(see Patent Document 1).

Furthermore, a method for manufacturing a transistor including an oxidesemiconductor in which a gate electrode is embedded in an opening isdisclosed (see Patent Documents 2 and 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2012-257187-   [Patent Document 2] Japanese Published Patent Application No.    2014-241407-   [Patent Document 3] Japanese Published Patent Application No.    2014-240833

DISCLOSURE OF INVENTION

An object is to provide a minute transistor. Another object is toprovide a transistor with low parasitic capacitance. Another object isto provide a transistor having high frequency characteristics. Anotherobject is to provide a transistor with favorable electriccharacteristics. Another object is to provide a transistor having stableelectric characteristics. Another object is to provide a transistorhaving low off-state current. Another object is to provide a noveltransistor. Another object is to provide a semiconductor deviceincluding the transistor. Another object is to provide a semiconductordevice that operates at high speed. Another object is to provide a novelsemiconductor device. Another object is to provide a module includingany of the above semiconductor devices. Another object is to provide anelectronic device including any of the above semiconductor devices orthe module.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

(1) One embodiment of the present invention is a semiconductor deviceincluding an oxide semiconductor, a first conductor, a second conductor,a third conductor, a first insulator, and a second insulator. The firstconductor overlaps with the oxide semiconductor with the first insulatorpositioned therebetween. The second insulator has an opening and a sidesurface of the second insulator overlaps with a side surface of thefirst conductor in the opening with the first insulator positionedtherebetween. Part of a surface of the second conductor and part of asurface of the third conductor are in contact with the first insulatorin the opening. The oxide semiconductor overlaps with the secondconductor and the third conductor.

(2) One embodiment of the present invention is the semiconductor devicedescribed in (1) where the first conductor is a multi-layer filmincluding a conductor that is less likely to allow passage of oxygen.

(3) One embodiment of the present invention is the semiconductor devicedescribed in (1) or (2) where each of the second conductor and the thirdconductor is a multi-layer film including a conductor that is lesslikely to allow passage of oxygen.

(4) One embodiment of the present invention is the semiconductor devicedescribed in any one of (1) to (3) where a third insulator overlaps withthe oxide semiconductor, and the third insulator contains at least oneof main component elements of the oxide semiconductor other than oxygen.

(5) One embodiment of the present invention is the semiconductor devicedescribed in any one of (1) to (4) where the first conductor has aregion serving a gate electrode of a transistor, and the transistor hasa gate line width of greater than or equal to 5 nm and less than orequal to 60 nm.

(6) One embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming a second insulator over a first insulator; forming an oxidesemiconductor over the second insulator; forming a first conductor overthe oxide semiconductor; etching part of the first conductor, part ofthe oxide semiconductor, and part of the second insulator to form amulti-layer film including the first conductor, the oxide semiconductor,and the second insulator; forming a third insulator over the firstinsulator and the multi-layer film; forming an opening exposing theoxide semiconductor in the third insulator and the first conductor toseparate the first conductor into a first conductor layer and a secondconductor layer; etching part of the third insulator to expand theopening and to expose a top surface of the first conductor layer and atop surface of the second conductor layer; forming a fourth insulatorover the third insulator, the first conductor layer, the secondconductor layer, and the oxide semiconductor; forming a fifth insulatorover the fourth insulator; forming a third conductor over the fifthinsulator; and performing chemical mechanical polishing on the thirdconductor, the fifth insulator, and the fourth insulator to expose thethird insulator. The second insulator and the fourth insulator containat least one of main component elements of the oxide semiconductor otherthan oxygen.

(7) One embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming a second insulator over a first insulator; forming an oxidesemiconductor over the second insulator; forming a first conductor overthe oxide semiconductor; etching part of the first conductor, part ofthe oxide semiconductor, and part of the second insulator to form amulti-layer film including the first conductor, the oxide semiconductor,and the second insulator; forming a third insulator over the firstinsulator and the multi-layer film; forming an opening exposing theoxide semiconductor in the third insulator and the first conductor toseparate the first conductor into a first conductor layer and a secondconductor layer; etching part of the third insulator to expand theopening and to expose a top surface of the first conductor layer and atop surface of the second conductor layer; forming a fourth insulatorover the third insulator, the first conductor layer, the secondconductor layer, and the oxide semiconductor; forming a fifth insulatorover the fourth insulator; forming a third conductor over the fifthinsulator; performing chemical mechanical polishing on the thirdconductor, the fifth insulator, and the fourth insulator to expose thethird insulator; forming a sixth insulator over the third insulator andthe third conductor using plasma including oxygen to add the oxygen inthe plasma to the third insulator as excess oxygen; and performing heattreatment to move the excess oxygen to the oxide semiconductor. Thesecond insulator and the fourth insulator contain at least one of maincomponent elements of the oxide semiconductor other than oxygen.

(8) One embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming a second insulator over a first insulator; forming an oxidesemiconductor over the second insulator; forming a first conductor overthe oxide semiconductor; etching part of the first conductor, part ofthe oxide semiconductor, and part of the second insulator to form amulti-layer film including the first conductor, the oxide semiconductor,and the second insulator; forming a third insulator over the firstinsulator and the multi-layer film; forming, in the third insulator, anopening exposing the multi-layer film and an opening exposing the firstinsulator; forming an organic film over the first insulator, the thirdinsulator, a side surface of the third insulator, the multi-layer film,and the side surface of the multi-layer film; etching part of theorganic film with the organic film over the side surface of the thirdinsulator and the side surface of the multi-layer film left; etching thefirst conductor using the organic film left over at least the sidesurface of the third insulator as a mask to separate the first conductorinto a first conductor layer and a second conductor layer; removing theorganic film over the side surface of the third insulator and the sidesurface of the multi-layer film to form an opening reaching a surface ofthe first conductor layer and a surface of the second conductor layer;forming a fourth insulator over the third insulator, the first conductorlayer, the second conductor layer, and the oxide semiconductor; forminga fifth insulator over the fourth insulator; forming a third conductorover the fifth insulator; performing chemical mechanical polishing onthe third conductor, the fifth insulator, and the fourth insulator toexpose the third insulator; and forming a sixth insulator over the thirdinsulator and the third conductor. The second insulator and the fourthinsulator contain at least one of main component elements of the oxidesemiconductor other than oxygen.

(9) One embodiment of the present invention is a method formanufacturing a semiconductor device, including the following steps:forming a second insulator over a first insulator; forming an oxidesemiconductor over the second insulator; forming a first conductor overthe oxide semiconductor; etching part of the first conductor, part ofthe oxide semiconductor, and part of the second insulator to form amulti-layer film including the first conductor, the oxide semiconductor,and the second insulator; forming a third insulator over the firstinsulator and the multi-layer film; forming, in the third insulator, anopening exposing the multi-layer film and an opening exposing the firstinsulator; forming an organic film over the first insulator, the thirdinsulator, a side surface of the third insulator, the multi-layer film,and the side surface of the multi-layer film; etching part of theorganic film with the organic film over the side surface of the thirdinsulator and the side surface of the multi-layer film left; etching thefirst conductor using the organic film left over at least the sidesurface of the third insulator as a mask to separate the first conductorinto a first conductor layer and a second conductor layer; removing theorganic film over the side surface of the third insulator and the sidesurface of the multi-layer film to form an opening reaching a surface ofthe first conductor layer and a surface of the second conductor layer;forming a fourth insulator over the third insulator, the first conductorlayer, the second conductor layer, and the oxide semiconductor; forminga fifth insulator over the fourth insulator; forming a third conductorover the fifth insulator; performing chemical mechanical polishing onthe third conductor, the fifth insulator, and the fourth insulator toexpose the third insulator; forming a sixth insulator over the thirdinsulator and the third conductor using plasma including oxygen to addthe oxygen in the plasma to the third insulator as excess oxygen; andperforming heat treatment to transfer the excess oxygen to the oxidesemiconductor. The second insulator and the fourth insulator contain atleast one of main component elements of the oxide semiconductor otherthan oxygen.

(10) One embodiment of the present invention is the method formanufacturing the semiconductor device described in any one of (6) to(9), further including a step of forming a fourth conductor that is lesslikely to allow passage of oxygen than the first conductor, over thefirst conductor.

(11) One embodiment of the present invention is the method formanufacturing the semiconductor device described in any one of (6) to(10), further including a step of forming a fifth conductor that is lesslikely to allow passage of oxygen than the third conductor, below thethird conductor.

(12) One embodiment of the present invention is the method formanufacturing a semiconductor device described in any one of (8) to (11)where the organic film is formed by plasma using a gas containing carbonand halogen.

(13) One embodiment of the present invention is the method formanufacturing the semiconductor device described in any one of (6) to(12) where the sixth insulator is an insulator that is less likely toallow passage of oxygen than the first insulator, the third insulator,and the fifth insulator.

Note that in the semiconductor device of one embodiment of the presentinvention, the oxide semiconductor may be replaced with anothersemiconductor.

A minute transistor can be provided. Alternatively, a transistor withlow parasitic capacitance can be provided. Alternatively, a transistorhaving high frequency characteristics can be provided. Alternatively, atransistor with favorable electric characteristics can be provided.Alternatively, a transistor with stable electric characteristics can beprovided. Alternatively, a transistor with low off-state current can beprovided. Alternatively, a novel transistor can be provided.Alternatively, a semiconductor device including the transistor can beprovided. Alternatively, a semiconductor device which can operate athigh speed can be provided. Alternatively, a novel semiconductor devicecan be provided. A module including any of the above semiconductordevices can be provided. An electronic device including any of the abovesemiconductor devices or the module can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIGS. 2A and 2B are cross-sectional views each illustrating part of atransistor of one embodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating part of a transistor ofone embodiment of the present invention;

FIGS. 4A to 4C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIGS. 5A to 5C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIGS. 6A to 6C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIGS. 7A to 7C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIGS. 8A to 8C are a top view and cross-sectional views of a transistorof one embodiment of the present invention;

FIG. 9 is a cross-sectional view of a transistor of one embodiment ofthe present invention;

FIGS. 10A to 10D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS;

FIGS. 11A to 11D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS;

FIGS. 12A to 12C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD;

FIGS. 13A and 13B show electron diffraction patterns of a CAAC-OS;

FIG. 14 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation;

FIGS. 15A to 15C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 16A to 16C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 17A to 17C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 18A to 18C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 19A to 19C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 20A to 20C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 21A to 21C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 22A to 22C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 23A to 23C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 24A to 24C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 25A to 25C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 26A to 26C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 27A to 27C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 28A to 28C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 29A to 29C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 30A to 30C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 31A to 31C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 32A to 32C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 33A to 33C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 34A to 34C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 35A to 35C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 36A to 36C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 37A to 37C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor of one embodiment of the presentinvention;

FIGS. 38A and 38B are each a circuit diagram of a memory device of oneembodiment of the present invention;

FIG. 39 is a cross-sectional view of a semiconductor device of oneembodiment of the present invention;

FIG. 40 is a cross-sectional view of a semiconductor device of oneembodiment of the present invention;

FIGS. 41A to 41F are cross sectional views and circuit diagrams of asemiconductor device of one embodiment of the present invention;

FIG. 42 is a block diagram of a CPU of one embodiment of the presentinvention;

FIG. 43 is a circuit diagram of a memory element of one embodiment ofthe present invention;

FIGS. 44A and 44B are plan views of imaging devices;

FIGS. 45A and 45B are plan views of pixels of an imaging device;

FIGS. 46A and 46B are cross-sectional views of an imaging device;

FIGS. 47A and 47B are cross-sectional views of imaging devices;

FIG. 48 illustrates a configuration example of an RF tag;

FIGS. 49A to 49C are a circuit diagram, a top view, and across-sectional view of a semiconductor device of one embodiment of thepresent invention;

FIGS. 50A and 50B are a circuit diagram and a cross-sectional view of asemiconductor device of one embodiment of the present invention;

FIG. 51 illustrates a display module;

FIGS. 52A and 52B are perspective views illustrating a cross-sectionalstructure of a package using a lead frame interposer;

FIGS. 53A to 53E each illustrate an electronic device of one embodimentof the present invention;

FIGS. 54A to 54D each illustrate an electronic device of one embodimentof the present invention;

FIGS. 55A to 55C each illustrate an electronic device of one embodimentof the present invention; and

FIGS. 56A to 56F illustrate application examples of an RF tag of oneembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiment of the present invention will be described indetail with the reference to the drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Further, the present inventionis not construed as being limited to description of the embodiments andthe examples. In describing structures of the present invention withreference to the drawings, common reference numerals are used for thesame portions in different drawings. Note that the same hatched patternis applied to similar parts, and the similar parts are not especiallydenoted by reference numerals in some cases.

Note that the size, the thickness of films (layers), or regions indrawings is sometimes exaggerated for simplicity.

In this specification, for example, when the shape of an object isdescribed with the use of a term such as “diameter”, “grain size(diameter)”, “dimension”, “size”, or “width”, the term can be regardedas the length of one side of a minimal cube where the object fits, or anequivalent circle diameter of a cross section of the object. The term“equivalent circle diameter of a cross section of the object” refers tothe diameter of a perfect circle having the same area as that of thecross section of the object.

A voltage usually refers to a potential difference between a givenpotential and a reference potential (e.g., a ground potential (GND) or asource potential). A voltage can be referred to as a potential and viceversa.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. Therefore, for example,description can be made even when “first” is replaced with “second” or“third”, as appropriate. In addition, the ordinal numbers in thisspecification and the like are not necessarily the same as those whichspecify one embodiment of the present invention.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration of lower than 0.1 atomic % isan impurity. When an impurity is contained, the density of states (DOS)may be formed in a semiconductor, the carrier mobility may be decreased,or the crystallinity may be decreased, for example. In the case wherethe semiconductor is an oxide semiconductor, examples of an impuritywhich changes characteristics of the semiconductor include Group 1elements, Group 2 elements, Group 14 elements, Group 15 elements, andtransition metals other than the main components; specifically, thereare hydrogen (included in water), lithium, sodium, silicon, boron,phosphorus, carbon, and nitrogen, for example. In the case of an oxidesemiconductor, oxygen vacancy may be formed by entry of impurities suchas hydrogen. Further, in the case where the semiconductor is a siliconfilm, examples of an impurity which changes characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Note that the channel length refers to, for example, a distance betweena source (source region or source electrode) and a drain (drain regionor drain electrode) in a region where a semiconductor (or a portionwhere a current flows in a semiconductor when a transistor is on) and agate electrode overlap with each other or a region where a channel isformed in a top view of the transistor. In one transistor, channellengths in all regions are not necessarily the same. In other words, thechannel length of one transistor is not limited to one value in somecases. Therefore, in this specification, the channel length is any oneof values, the maximum value, the minimum value, or the average value ina region where a channel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions arenot necessarily the same. In other words, the channel width of onetransistor is not limited to one value in some cases. Therefore, in thisspecification, the channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is actually formed (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel formation region formed in a side surface of asemiconductor is increased in some cases. In that case, an effectivechannel width obtained when a channel is actually formed is greater thanan apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example,estimation of an effective channel width from a design value requires anassumption that the shape of a semiconductor is known. Therefore, in thecase where the shape of a semiconductor is not known accurately, it isdifficult to measure an effective channel width accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Furthermore, in this specification, in thecase where the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, the values may be different from those calculated using aneffective channel width in some cases.

Note that in this specification, the description “A has a shape suchthat an end portion extends beyond an end portion of B” may indicate,for example, the case where at least one of end portions of A ispositioned on an outer side of at least one of end portions of B in atop view or a cross-sectional view. Thus, the description “A has a shapesuch that an end portion extends beyond an end portion of B” can be readas the description “one end portion of A is positioned on an outer sideof one end portion of B in a top view,” for example.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “perpendicular” indicates that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly also includes the case where the angle is greaterthan or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

Embodiment 1 <Transistor Structure 1>

A structure of a transistor included in a semiconductor device of oneembodiment of the present invention will be described below.

FIGS. 1A to 1C are a top view and cross-sectional views of asemiconductor device of one embodiment of the present invention. FIG. 1Ais the top view, and FIGS. 1B and 1C are the cross-sectional views takenalong dashed-dotted lines A1-A2 and A3-A4 in FIG. 1A, respectively. Notethat for simplification of the drawing, some components are notillustrated in the top view in FIG. 1A. In FIGS. 1B and 1C, pathsthrough which excess oxygen passes are indicated by arrows.

In FIGS. 1B and 1C, the transistor includes an insulator 402 over asubstrate 400; an insulator 406 a over the insulator 402; asemiconductor 406 b over the insulator 406 a; conductors 416 a 1 and 416a 2 each including a region in contact with a top surface of thesemiconductor 406 b; an insulator 410 in contact with top surfaces ofthe conductors 416 a 1 and 416 a 2; an insulator 406 c in contact withthe top surface of the semiconductor 406 b; an insulator 412 over theinsulator 406 c; a conductor 404 over the semiconductor 406 b with theinsulator 412 and the insulator 406 c positioned therebetween; aninsulator 408 over the insulator 410, the conductor 404, the insulator412, and the conductor 404 c; an opening reaching the conductor 404through the insulator 408; an opening reaching the conductor 416 a 1through the insulator 408 and the insulator 410; an opening reaching theconductor 416 a 2 through the insulator 408 and the insulator 410; aconductor 437, a conductor 431, and a conductor 429 that are embedded inthe openings; a conductor 438 over the insulator 408, which includes aregion in contact with the conductor 437; a conductor 432 over theinsulator 408, which includes a region in contact with the conductor431; and a conductor 430 over the insulator 408, which includes a regionin contact with the conductor 429.

Note that the semiconductor 406 b includes a region 407 in which the topsurface of the semiconductor 406 b is in contact with the conductors 416a 1 and 416 a 2.

In the transistor, the conductor 404 serves as a gate electrode.Furthermore, the conductor 404 can have a stacked structure including aconductor that hardly allows oxygen to pass therethrough. For example,when the conductor that hardly allows oxygen to pass therethrough isformed as a lower layer, an increase in the electric resistance valuedue to oxidation of the conductor 404 can be prevented. The insulator412 serves as a gate insulator.

The conductors 416 a 1 and 416 a 2 function as a source electrode and adrain electrode of the transistor. The conductors 416 a 1 and 416 a 2each can have a stacked structure including a conductor that hardlyallows oxygen to pass therethrough. For example, when the conductor thathardly allows oxygen to pass therethrough is formed as an upper layer,an increase in the electric resistance value due to oxidation of theconductors 416 a 1 and 416 a 2 can be prevented. Note that the electricresistance values of the conductors can be measured by a two-terminalmethod or the like.

Therefore, the resistance of the semiconductor 406 b can be controlledby a potential applied to the conductor 404. That is, conduction ornon-conduction between the conductors 416 a 1 and 416 a 2 can becontrolled by the potential applied to the conductor 404.

As illustrated in FIGS. 1B and 1C, the top surface of the semiconductor406 b is in contact with the conductors 416 a 1 and 416 a 2. Inaddition, the insulator 406 a and the semiconductor 406 b can beelectrically surrounded by an electric field of the conductor 404serving as the gate electrode. A structure in which a semiconductor iselectrically surrounded by an electric field of a gate electrode isreferred to as a surrounded channel (s-channel) structure. Therefore, achannel is formed in the entire semiconductor 406 b (bulk) in somecases. In the s-channel structure, a large amount of current can flowbetween a source and a drain of the transistor, so that an on-statecurrent can be increased. In addition, since the insulator 406 a and thesemiconductor 406 b are surrounded by the electric field of theconductor 404, an off-state current can be decreased.

The transistor can also be referred to as a trench-gate self-aligneds-channel FET (TGSA s-channel FET) because the region serving as a gateelectrode is formed in a self-aligned manner to fill the opening formedin the insulator 410 and the like.

FIGS. 2A and 2B are each an enlarged view of a central portion of thetransistor. In FIG. 2A, the length of a region where a bottom surface ofthe conductor 404 serving as a gate electrode and the top surface of thesemiconductor 406 b are parallel to each other with the insulator 412and the insulator 406 c positioned therebetween, is referred to as agate line width 404 w. In the opening in the insulator 410, the lengthof a region which is in contact with the insulator 406 c and the topsurface of the conductor 416 a 1 and is parallel to a bottom surface ofthe conductor 416 a 1 is a first opening width 416 w 1. Furthermore, inthe opening in the insulator 410, the length of a region which is incontact with the insulator 406 c and the top surface of the conductor416 a 2 and is parallel to a bottom surface of the conductor 416 a 2 isa second opening width 416 w 2. In the transistor of one embodiment ofthe present invention, the length of the opening reaching thesemiconductor 406 b is not changed regardless of the first opening width416 w 1 and the second opening width 416 w 2. Furthermore, the gate linewidth 404 w can be smaller than the opening reaching the semiconductor406 b. That is, the gate line width 404 w can be smaller than theminimum feature size. Specifically, the gate line width 404 w can begreater than or equal to 5 nm and less than or equal to 60 nm,preferably greater than or equal to 5 nm and less than or equal to 30nm.

When an electric field from the gate electrode is blocked by otherconductors, switching characteristics of the transistor may be degraded.In the transistor, the positional relationship between the conductor 404and the conductors 416 a 1 and 416 a 2 is changed by the thicknesses ofthe insulator 406 c and the insulator 412. That is, the relationshipbetween the thicknesses of the conductors 416 a 1 and 416 a 2 serving asthe source electrode and the drain electrode and the thickness of theinsulator 412 serving as the gate insulator affects electriccharacteristics of the transistor.

In FIG. 2B, the thickness of the insulator 412 in a region between theconductor 416 a 1 and the conductor 416 a 2 is denoted by 412 h. Thethickness of the conductor 416 a 1 or the conductor 416 a 2 is denotedby 416 h. It is preferable that the thickness 412 h be less than orequal to the thickness 416 h because the electric field from the gateelectrode is applied to the entire channel formation region, making theoperation of the transistor favorable. The thickness 412 h is set toless than or equal to 30 nm, preferably less than or equal to 10 nm.

The transistor can have a structure in which the thickness 416 h has asmall value. An end portion of the conductor 416 a 1 has a region facingthe conductor 404 with the insulator 406 c and the insulator 412positioned therebetween. An end portion of the conductor 416 a 2 has aregion facing the conductor 404 with the insulator 406 c and theinsulator 412 positioned therebetween. The vicinities of these regionsare denoted by portions surrounded by dotted circles in FIG. 2B;however, the area of these regions can be further reduced. Thus,parasitic capacitance of these regions in the transistor is reduced.

Note that the transistor is surrounded by an insulator which has afunction of blocking oxygen and impurities such as hydrogen, wherebystable electric characteristics can be obtained. For example, as theinsulator 408, an insulator which has a function of blocking oxygen andimpurities such as hydrogen may be used.

An insulator which has a function of blocking oxygen and impurities suchas hydrogen may be formed to have a single-layer structure or astacked-layer structure including an insulator containing, for example,boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum.

Furthermore, for example, the insulator 408 may be formed using aluminumoxide, magnesium oxide, silicon nitride oxide, silicon nitride, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide. Note that theinsulator 408 preferably contains aluminum oxide. For example, when theinsulator 408 is formed using plasma including oxygen, oxygen can beadded to the insulator 410 serving as a base layer of the insulator 408.Alternatively, oxygen can be added to side surfaces of the insulator412. The added oxygen becomes excess oxygen in the insulator 410 or theinsulator 412. When the insulator 408 contains aluminum oxide, entry ofimpurities such as hydrogen into the semiconductor 406 b can beinhibited. Furthermore, when the insulator 408 contains aluminum oxide,for example, outward diffusion of excess oxygen added to the insulator410 and the insulator 412 can be reduced.

The insulator 402 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including an insulator containingboron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. For example, the insulator402 preferably includes silicon oxide or silicon oxynitride.

Note that the insulator 410 preferably includes an insulator with lowdielectric constant. For example, the insulator 410 preferably includessilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, silicon oxide to which fluorine is added, silicon oxide towhich carbon is added, silicon oxide to which carbon and nitrogen areadded, porous silicon oxide, a resin, or the like. Alternatively, theinsulator 410 preferably has a stacked structure of a resin and siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,silicon oxide to which fluorine is added, silicon oxide to which carbonis added, silicon oxide to which carbon and nitrogen are added, orporous silicon oxide. When silicon oxide or silicon oxynitride, which isthermally stable, is combined with a resin, the stacked-layer structurecan have thermal stability and low dielectric constant. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, and acrylic.

The insulator 412 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including an insulator containingboron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. For example, the insulator412 preferably includes silicon oxide or silicon oxynitride.

Note that the insulator 412 preferably contains an insulator with a highdielectric constant. For example, the insulator 412 preferably includesgallium oxide, hafnium oxide, oxide including aluminum and hafnium,oxynitride including aluminum and hafnium, oxide including silicon andhafnium, oxynitride including silicon and hafnium, or the like. Theinsulator 412 preferably has a stacked-layer structure including siliconoxide or silicon oxynitride and an insulator with a high dielectricconstant. Because silicon oxide and silicon oxynitride have thermalstability, combination of silicon oxide or silicon oxynitride with aninsulator with a high dielectric constant allows the stacked-layerstructure to be thermally stable and have a high dielectric constant.For example, when an aluminum oxide, a gallium oxide, or a hafnium oxideof the insulator 412 is on the insulator 406 c side, entry of siliconincluded in the silicon oxide or the silicon oxynitride into thesemiconductor 406 b can be suppressed. When silicon oxide or siliconoxynitride is on the insulator 406 c side, for example, trap centersmight be formed at the interface between aluminum oxide, gallium oxide,or hafnium oxide and silicon oxide or silicon oxynitride. The trapcenters can shift the threshold voltage of the transistor in thepositive direction by trapping electrons in some cases.

Each of the conductors 416 a 1 and 416 a 2 may be formed to have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, platinum, silver, indium, tin, tantalum, andtungsten. Alternatively, a film of an alloy or compound containing theabove element may be used: a conductor containing aluminum, a conductorcontaining copper and titanium, a conductor containing copper andmanganese, a conductor containing indium, tin, and oxygen, a conductorcontaining titanium and nitrogen, or the like may be used.Alternatively, each of the conductors 416 a 1 and 416 a 2 is preferablya stacked-layer film of a conductor containing nitrogen and tantalumcontaining nitrogen that is less likely to transmit oxygen.

The conductor 404 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including a conductor containingone or more kinds of boron, nitrogen, oxygen, fluorine, silicon,phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel,copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium,silver, indium, tin, tantalum, and tungsten. Alternatively, a film of analloy or compound containing the above element may be used: a conductorcontaining aluminum, a conductor containing copper and titanium, aconductor containing copper and manganese, a conductor containingindium, tin, and oxygen, a conductor containing titanium and nitrogen,or the like may be used. Alternatively, each of the conductors 416 a 1and 416 a 2 is preferably a stacked-layer film of a conductor containingnitrogen and tantalum containing nitrogen that is less likely totransmit oxygen.

Each of the conductors 429, 430, 431, 432, 437, and 438 may be formed tohave a single-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten.Alternatively, a film of an alloy or compound containing the aboveelement may be used: a conductor containing aluminum, a conductorcontaining copper and titanium, a conductor containing copper andmanganese, a conductor containing indium, tin, and oxygen, a conductorcontaining titanium and nitrogen, or the like may be used.Alternatively, each of the conductors 416 a 1 and 416 a 2 is preferablya stacked-layer film of a conductor containing nitrogen and tantalumcontaining nitrogen that is less likely to transmit oxygen.

As the semiconductor 406 b, an oxide semiconductor is preferably used.However, silicon (including strained silicon), germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, an organic semiconductor, or the likecan be used in some cases.

The insulator 406 a and the insulator 406 c are desirably oxidesincluding one or more, or two or more elements other than oxygenincluded in the semiconductor 406 b. However, silicon (includingstrained silicon), germanium, silicon germanium, silicon carbide,gallium arsenide, aluminum gallium arsenide, indium phosphide, galliumnitride, an organic semiconductor, or the like can be used in somecases.

<Transistor Structure 2>

A transistor having a structure different from that in FIGS. 1A to 1Cwill be described with reference to FIGS. 4A to 4C. FIGS. 4A to 4C are atop view and cross-sectional views of a semiconductor device of oneembodiment of the present invention. FIG. 4A is the top view, and FIGS.4B and 4C are the cross-sectional views taken along dashed-dotted linesA1-A2 and A3-A4 in FIG. 4A, respectively. Note that for simplificationof the drawing, some components are not illustrated in the top view inFIG. 4A.

In FIGS. 4B and 4C, the transistor includes an insulator 401 over thesubstrate 400; a conductor 310 over the insulator 401; an insulator 301in contact with top surfaces of the insulator 401 and the conductor 310and side surfaces of the conductor 310; an insulator 303 over theinsulator 301; the insulator 402 over the insulator 303; the insulator406 a over the insulator 402; the semiconductor 406 b over the insulator406 a; the conductors 416 a 1 and 416 a 2 each including the region incontact with the top surface of the semiconductor 406 b; the insulator410 in contact with the top surfaces of the conductors 416 a 1 and 416 a2; the insulator 406 c in contact with the top surface of thesemiconductor 406 b; the insulator 412 over the insulator 406 c; theconductor 404 placed over the semiconductor 406 b with the insulator 412and the insulator 406 c positioned therebetween; the insulator 408 overthe insulator 410, the conductor 404, the insulator 412, and theinsulator 406 c; the opening reaching the conductor 404 through theinsulator 408; the opening reaching the conductor 416 a 1 through theinsulator 408 and the insulator 410; the opening reaching the conductor416 a 2 through the insulator 408 and the insulator 410; the conductor437, the conductor 431, and the conductor 429 that are embedded in theopenings; the conductor 438 over the insulator 408, which includes theregion in contact with the conductor 437; the conductor 432 over theinsulator 408, which includes the region in contact with the conductor431; and the conductor 430 over the insulator 408, which includes theregion in contact with the conductor 429.

Note that the semiconductor 406 b includes the region 407 in which thetop surface of the semiconductor 406 b is in contact with the conductor416 a 1 and the conductor 416 a 2.

In the transistor, the conductor 404 serves as a first gate electrode.Furthermore, the conductor 404 can have a stacked structure including aconductor that hardly allows oxygen to pass therethrough. For example,when the conductor that hardly allows oxygen to pass therethrough isformed as a lower layer, an increase in the electric resistance valuedue to oxidation of the conductor 404 can be prevented. The insulator412 serves as the gate insulator.

The conductors 416 a 1 and 416 a 2 function as a source electrode and adrain electrode of the transistor. The conductors 416 a 1 and 416 a 2can each have a stacked structure including a conductor that hardlyallows oxygen to pass therethrough. For example, when the conductor thathardly allows oxygen to pass therethrough is formed as an upper layer,an increase in the electric resistance value due to oxidation of theconductors 416 a 1 and 416 a 2 can be prevented.

Therefore, the resistance of the semiconductor 406 b can be controlledby a potential applied to the conductor 404. That is, conduction ornon-conduction between the conductors 416 a 1 and 416 a 2 can becontrolled by the potential applied to the conductor 404.

The conductor 310 functions as a second gate electrode. The conductor310 can be a multilayer film including a conductive film that hardlyallows oxygen to pass therethrough. The use of a multilayer filmincluding a conductive film that hardly allows oxygen to passtherethrough can prevent a decrease in conductivity due to oxidation ofthe conductor 310. The insulators 301, 303, and 402 serve as a secondgate insulator. The threshold voltage of the transistor can becontrolled by a potential applied to the conductor 310. Furthermore, bythe potential applied to the conductor 310, electrons are injected tothe insulator 303 and thus the threshold voltage of the transistor canbe controlled. The first gate electrode and the second gate electrodeare electrically connected to each other, whereby a high on-statecurrent can be obtained. Note that the functions of the first gateelectrode and the second gate electrode may be replaced with each other.

FIG. 9 illustrates an example in which the first gate electrode and thesecond gate electrode are electrically connected. In the openingreaching the conductor 404 through the insulator 408, the conductor 440is embedded, and a top surface of the conductor 440 is electricallyconnected to a conductor 444 formed over the insulator 408. In anopening reaching the conductor 310 through the insulator 408, theinsulator 410, the insulator 402, the insulator 303, and the insulator301, the conductor 442 is embedded, and a top surface of the conductor442 and the conductor 444 are electrically connected. That is, theconductor 404 serving as the first gate electrode is electricallyconnected to the conductor 310 serving as the second gate electrodethrough the conductors 440, 444, and 442.

For example, the insulator 401 may be formed of aluminum oxide,magnesium oxide, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide. Note that theinsulator 401 preferably includes aluminum oxide or silicon nitride. Theinsulator 401 including aluminum oxide or silicon nitride can suppressentry of impurities such as hydrogen into the semiconductor 406 b, andcan reduce outward diffusion of oxygen, for example

The insulator 301 may be formed to have, for example, a single-layerstructure or a stacked-layer structure including an insulator containingboron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. For example, the insulator301 preferably includes silicon oxide or silicon oxynitride.

The insulator 303 may serves as an electron injection layer, forexample. The insulator 303 may be formed to have, for example, asingle-layer structure or a stacked-layer structure including aninsulator containing boron, carbon, nitrogen, oxygen, fluorine,magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium,germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, ortantalum. For example, the insulator 303 preferably includes siliconnitride, hafnium oxide, or aluminum oxide.

Each of the conductors 310, 440, 442, and 444 may be formed to have asingle-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten.Alternatively, a film of an alloy or compound containing the aboveelement may be used: a conductor containing aluminum, a conductorcontaining copper and titanium, a conductor containing copper andmanganese, a conductor containing indium, tin, and oxygen, a conductorcontaining titanium and nitrogen, or the like may be used.

This transistor is different from the transistor in FIGS. 1A to 1C inthat the conductor 310 serving as the second gate electrode is included.For the same components as those in FIGS. 1A to 1C, refer to the abovedescription.

<Transistor Structure 3>

A transistor having a structure different from that in FIGS. 1A to 1Cwill be described with reference to FIGS. 5A to 5C. FIGS. 5A to 5C are atop view and cross-sectional views of a semiconductor device of oneembodiment of the present invention. FIG. 5A is the top view and FIGS.5B and 5C are the cross-sectional views taken along dashed-dotted linesA1-A2 and A3-A4 in FIG. 5A, respectively. Note that for simplificationof the drawing, some components in the top view in FIG. 5A are notillustrated.

This transistor is different from the transistor in FIGS. 1A to 1C inthe shapes of the conductors 416 a 1 and 416 a 2. Details thereof aredescribed below with reference to FIG. 3 .

FIG. 3 is an enlarged view of a central portion of the transistor inFIGS. 5A to 5C. In the case where an angle formed by a plane which isparallel to the substrate and a side surface of the conductor 416 a 1where the conductor 416 a 1 is in contact with the insulator 406 c is ataper angle 446, the taper angle 446 is less than or equal to 90°. Anangle formed by a plane which is parallel to the substrate and a sidesurface of the conductor 416 a 2 where the conductor 416 a 2 is incontact with the insulator 406 c is a taper angle 447, the taper angle447 is less than or equal to 90°. The taper angles 446 and 447 is lessthan or equal to 90°; coverage by the insulator 406 c formed on the sidesurfaces of the conductors 416 a 1 and 416 a 2 and the insulator 412serving as the gate insulator is improved. When the coverage by theinsulator 406 c and the insulator 412 is improved, leakage currentflowing through the conductor 404 serving as a gate electrode and theconductor 416 a 1 or 416 a 2 serving as a source electrode or a drainelectrode can be kept low. Alternatively, leakage current flowingthrough the conductor 404 serving as a gate electrode and thesemiconductor 406 b having a channel formation region can be kept low.The taper angles 446 and 447 are each greater than or equal to 10° andless than 90°, preferably, greater than or equal to 30° and less than orequal to 80°.

The length of the opening reaching the semiconductor 406 b is notchanged regardless of the first opening width 416 w 1 and the secondopening width 416 w 2. Furthermore, the gate line width 404 w can besmaller than the opening reaching the semiconductor 406 b. That is, thegate line width 404 w can be smaller than the minimum feature size.Specifically, the gate line width 404 w can be greater than or equal to5 nm and less than or equal to 60 nm, preferably greater than or equalto 5 nm and less than or equal to 30 nm. For the other components, thedescription of the transistor in FIGS. 1A to 1C is referred to.

<Transistor Structure 4>

A transistor having a structure different from that in FIGS. 1A to 1Cwill be described here with reference to FIGS. 6A to 6C. FIGS. 6A to 6Care a top view and cross-sectional views of a semiconductor device ofone embodiment of the present invention. FIG. 6A is the top view, andFIGS. 6B and 6C are the cross-sectional views taken along dashed-dottedlines A1-A2 and A3-A4 in FIG. 6A, respectively. Note that forsimplification of the drawing, some components are not illustrated inthe top view in FIG. 6A.

The transistor in FIGS. 6A to 6C has a structure in which the conductor310 having a function of the second gate electrode is added to thetransistor in FIGS. 5A to 5C. For the same components, refer to theabove description.

<Transistor Structure 5>

A transistor having a structure different from that in FIGS. 4A to 4Cwill be described with reference to FIGS. 7A to 7C. FIGS. 7A to 7C are atop view and cross-sectional views of a semiconductor device of oneembodiment of the present invention. FIG. 7A is the top view and FIGS.7B and 7C are the cross-sectional views taken along dashed-dotted linesA1-A2 and A3-A4 in FIG. 7A, respectively. Note that for simplificationof the drawing, some components are not illustrated in the top view inFIG. 7A.

This transistor is different from the transistor in FIGS. 4A to 4C inthat the transistor has a plurality of channel formation regions.Although the transistor has three channel formation regions as anexample in FIGS. 7A to 7C, the number of channel formation regions isnot limited to this. For the other components, the description of thetransistor in FIGS. 4A to 4C is referred to.

<Transistor Structure 6>

A transistor having a structure different from that in FIGS. 4A to 4Cwill be described with reference to FIGS. 8A to 8C. FIGS. 8A to 8C are atop view and cross-sectional views of a semiconductor device of oneembodiment of the present invention. FIG. 8A is the top view, and FIGS.8B and 8C are the cross-sectional views taken along dashed-dotted linesA1-A2 and A3-A4 in FIG. 8A, respectively. Note that for simplificationof the drawing, some components are not illustrated in the top view inFIG. 8A.

This transistor has a channel that is larger than the gate line width404 w in FIG. 2A. For the other components, the description of thetransistor in FIGS. 4A to 4C is referred to.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 2 <Structure of Oxide Semiconductor>

A structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 10A shows ahigh-resolution TEM image of a cross section of the CAAC-OS that isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 10B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 10A. FIG. 18B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 10B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 10C. The size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm, and the size of a spacecaused by tilt of the pellets is approximately 0.8 nm. Therefore, thepellet can also be referred to as a nanocrystal (nc). Furthermore, theCAAC-OS can also be referred to as an oxide semiconductor includingc-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 10D). The part in which the pellets are tilted as observed inFIG. 10C corresponds to a region 5161 shown in FIG. 10D.

FIG. 11A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 11B, 11C, and 11D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 11A,respectively. FIGS. 11B, 11C, and 11D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 12A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray beam is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (0 scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (0 axis), as shown in FIG. 12B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 12C, six peaks that are derivedfrom crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 13A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 13B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 13B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 13B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.The second ring in FIG. 13B is considered to be derived from the (110)plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacanciesin the oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferablylower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Suchan oxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which it is difficult to observe a crystal part clearly in ahigh-resolution TEM image. In most cases, the size of a crystal partincluded in the nc-OS is greater than or equal to 1 nm and less than orequal to 10 nm, or greater than or equal to 1 nm and less than or equalto 3 nm. Note that an oxide semiconductor including a crystal part whosesize is greater than 10 nm and less than or equal to 100 nm is sometimesreferred to as a microcrystalline oxide semiconductor. In ahigh-resolution TEM image of the nc-OS, for example, it is difficult toobserve a crystal grain boundary clearly in some cases. Note that thereis a possibility that the origin of the nanocrystal is the same as thatof a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakthat shows a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of spots areshown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.

Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region in which a crystal partis not observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 14 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 14 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 14 , acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 14 , the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a multilayerfilm including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

An oxide which can be used as the insulator 406 a, the semiconductor 406b, the insulator 406 c, or the like is described.

The semiconductor 406 b is an oxide semiconductor containing indium, forexample. The oxide semiconductor 406 b can have high carrier mobility(electron mobility) by containing indium, for example. The semiconductor406 b preferably contains an element M. The element M is preferablyaluminum, gallium, yttrium, tin, or the like. Other elements which canbe used as the element M are boron, silicon, titanium, iron, nickel,germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium,tantalum, tungsten, and the like. Note that two or more of the aboveelements may be used in combination as the element M. The element M isan element having high bonding energy with oxygen, for example. Theelement M is an element whose bonding energy with oxygen is higher thanthat of indium. The element M is an element that can increase the energygap of the oxide semiconductor, for example. Furthermore, thesemiconductor 406 b preferably contains zinc. When the oxidesemiconductor contains zinc, the oxide semiconductor is easilycrystallized, in some cases.

Note that the semiconductor 406 b is not limited to the oxidesemiconductor containing indium. The semiconductor 406 b may be, forexample, an oxide semiconductor which does not contain indium andcontains zinc, an oxide semiconductor which does not contain indium andcontains gallium, or an oxide semiconductor which does not containindium and contains tin, e.g., a zinc tin oxide, a gallium tin oxide, ora gallium oxide.

For the semiconductor 406 b, an oxide with a wide energy gap may beused, for example. For example, the energy gap of the semiconductor 406b is greater than or equal to 2.5 eV and less than or equal to 4.2 eV,preferably greater than or equal to 2.8 eV and less than or equal to 3.8eV, and further preferably greater than or equal to 3 eV and less thanor equal to 3.5 eV.

For example, the insulator 406 a and the insulator 406 c are oxidesincluding one or more, or two or more elements other than oxygenincluded in the semiconductor 406 b. Since the insulator 406 a and theinsulator 406 c each include one or more, or two or more elements otherthan oxygen included in the semiconductor 406 b, an interface state isless likely to be formed at the interface between the insulator 406 aand the semiconductor 406 b and the interface between the semiconductor406 b and the insulator 406 c.

The case where the insulator 406 a, the semiconductor 406 b, and theinsulator 406 c contain indium is described. In the case of using anIn-M-Zn oxide as the insulator 406 a, when a summation of In and M isassumed to be 100 atomic %, the proportions of In and M are preferablyset to be less than 50 atomic % and greater than 50 atomic %,respectively, further preferably less than 25 atomic % and greater than75 atomic %, respectively. In the case of using an In-M-Zn oxide as thesemiconductor 406 b, when the summation of In and M is assumed to be 100atomic %, the proportions of In and M are preferably set to be greaterthan 25 atomic % and less than 75 atomic %, respectively, furtherpreferably greater than 34 atomic % and less than 66 atomic %,respectively. In the case of using an In-M-Zn oxide as the insulator 406c, when the summation of In and M is assumed to be 100 atomic %, theproportions of In and M are preferably set to be less than 50 atomic %and greater than 50 atomic %, respectively, further preferably less than25 atomic % and greater than 75 atomic %, respectively. Note that theinsulator 406 c may be an oxide that is a type the same as that of theinsulator 406 a.

As the semiconductor 406 b, an oxide having an electron affinity higherthan those of the insulators 406 a and 406 c is used. For example, asthe semiconductor 406 b, an oxide having an electron affinity higherthan those of the insulators 406 a and 406 c by 0.07 eV or higher and1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower,further preferably 0.15 eV or higher and 0.4 eV or lower is used. Notethat the electron affinity refers to an energy gap between the vacuumlevel and the bottom of the conduction band.

An indium gallium oxide has a small electron affinity and a highoxygen-blocking property. Therefore, the insulator 406 c preferablyincludes an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)]is, for example, higher than or equal to 70%, preferably higher than orequal to 80%, further preferably higher than or equal to 90%.

Note that the insulator 406 a and/or the insulator 406 c may be galliumoxide. For example, when gallium oxide is used as the insulator 406 c,leakage current between the conductor 404 and the conductor 416 a 1 orthe conductor 416 a 2 can be reduced. In other words, the off-statecurrent of the transistor can be reduced.

At this time, when a gate voltage is applied, a channel is formed in thesemiconductor 406 b having the highest electron affinity in theinsulator 406 a, the semiconductor 406 b, and the insulator 406 c.

Here, in some cases, there is a mixed region of the insulator 406 a andthe semiconductor 406 b between the insulator 406 a and thesemiconductor 406 b. Furthermore, in some cases, there is a mixed regionof the semiconductor 406 b and the insulator 406 c between thesemiconductor 406 b and the insulator 406 c. The mixed region has a lowdensity of interface states. For that reason, the stack of the insulator406 a, the semiconductor 406 b, and the insulator 406 c has a bandstructure where energy at each interface and in the vicinity of theinterface is changed continuously (continuous junction).

At this time, electrons move mainly in the semiconductor 406 b, not inthe insulator 406 a and the insulator 406 c. Thus, when the interfacestate density at the interface between the insulator 406 a and thesemiconductor 406 b and the interface state density at the interfacebetween the semiconductor 406 b and the insulator 406 c are decreased,electron movement in the semiconductor 406 b is less likely to beinhibited and the on-sate current of the transistor can be increased.

In the case where the transistor has an s-channel structure, a channelis formed in the whole of the semiconductor 406 b. Therefore, as thesemiconductor 406 b has a larger thickness, a channel region becomeslarger. In other words, the thicker the semiconductor 406 b is, thelarger the on-state current of the transistor is. For example, thesemiconductor 406 b has a region with a thickness of greater than orequal to 20 nm, preferably greater than or equal to 40 nm, morepreferably greater than or equal to 60 nm, still more preferably greaterthan or equal to 100 nm. Note that the semiconductor 406 b has a regionwith a thickness of, for example, less than or equal to 300 nm,preferably less than or equal to 200 nm, or more preferably less than orequal to 150 nm because the productivity of the semiconductor deviceincluding the transistor might be decreased.

Moreover, the thickness of the insulator 406 c is preferably as small aspossible to increase the on-state current of the transistor. Thethickness of the insulator 406 c is less than 10 nm, preferably lessthan or equal to 5 nm, more preferably less than or equal to 3 nm, forexample. Meanwhile, the insulator 406 c has a function of blocking entryof elements other than oxygen (such as hydrogen and silicon) included inthe adjacent insulator into the semiconductor 406 b where a channel isformed. For this reason, it is preferable that the insulator 406 c havea certain thickness. The thickness of the insulator 406 c is greaterthan or equal to 0.3 nm, preferably greater than or equal to 1 nm,further preferably greater than or equal to 2 nm, for example. Theinsulator 406 c preferably has an oxygen blocking property to suppressoutward diffusion of oxygen released from the insulator 402 and thelike.

To improve reliability, preferably, the thickness of the insulator 406 ais large and the thickness of the insulator 406 c is small. For example,the insulator 406 a has a region with a thickness, for example, greaterthan or equal to 10 nm, preferably greater than or equal to 20 nm,further preferably greater than or equal to 40 nm, still furtherpreferably greater than or equal to 60 nm. When the thickness of theinsulator 406 a is made large, a distance from an interface between theadjacent insulator and the insulator 406 a to the semiconductor 406 b inwhich a channel is formed can be large. Since the productivity of thesemiconductor device including the transistor might be decreased, theinsulator 406 a has a region with a thickness, for example, less than orequal to 200 nm, preferably less than or equal to 120 nm, furtherpreferably less than or equal to 80 nm.

For example, silicon in the oxide semiconductor might serve as a carriertrap or a carrier generation source. Therefore, the siliconconcentration of the semiconductor 406 b is preferably as low aspossible. For example, a region in which the concentration of siliconwhich is measured by secondary ion mass spectrometry (SIMS) is lowerthan 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, orfurther preferably lower than 2×10¹⁸ atoms/cm³ is provided between thesemiconductor 406 b and the insulator 406 a. A region with a siliconconcentration lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸atoms/cm³, further preferably lower than 2×10¹⁸ atoms/cm³ which ismeasured by SIMS is provided between the semiconductor 406 b and theinsulator 406 c.

It is preferable to reduce the concentration of hydrogen in theinsulator 406 a and the insulator 406 c in order to reduce theconcentration of hydrogen in the semiconductor 406 b. The insulator 406a and the insulator 406 c each have a region in which the concentrationof hydrogen measured by SIMS is lower than or equal to 2×10²⁰ atoms/cm³,preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferablylower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lowerthan or equal to 5×10¹⁸ atoms/cm³. It is preferable to reduce theconcentration of nitrogen in the insulator 406 a and the insulator 406 cin order to reduce the concentration of nitrogen in the semiconductor406 b. The insulator 406 a and the insulator 406 c each have a region inwhich the concentration of nitrogen measured by SIMS is lower than5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³,further preferably lower than or equal to 1×10¹⁸ atoms/cm³, stillfurther preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Note that when copper enters the oxide semiconductor, an electron trapmight be generated. The electron trap might shift the threshold voltageof the transistor in the positive direction. Therefore, theconcentration of copper on the surface of or in the semiconductor 406 bis preferably as low as possible. For example, the semiconductor 406 bpreferably has a region in which the copper concentration is lower thanor equal to 1×10¹⁹ atoms/cm³, lower than or equal to 5×10¹⁸ atoms/cm³,or lower than or equal to 1×10¹⁸ atoms/cm³.

The above three-layer structure is an example. For example, a two-layerstructure without the insulator 406 a or the insulator 406 c may beemployed. Alternatively, a four-layer structure in which any one of theinsulators or the semiconductors described as examples of the insulator406 a, the semiconductor 406 b, and the insulator 406 c is providedbelow or over the insulator 406 a or below or over the insulator 406 cmay be employed. Alternatively, an n-layer structure (n is an integer of5 or more) in which any one of the insulators or the semiconductorsdescribed as examples of the insulator 406 a, the semiconductor 406 b,and the insulator 406 c is provided at two or more of the followingpositions: over the insulator 406 a, below the insulator 406 a, over theinsulator 406 c, and below the insulator 406 c.

As the substrate 400, an insulator substrate, a semiconductor substrate,or a conductor substrate may be used, for example. As the insulatorsubstrate, a glass substrate, a quartz substrate, a sapphire substrate,a stabilized zirconia substrate (e.g., an yttria-stabilized zirconiasubstrate), or a resin substrate is used, for example. As thesemiconductor substrate, a single material semiconductor substrate ofsilicon, germanium, or the like or a compound semiconductor substratecontaining silicon carbide, silicon germanium, gallium arsenide, indiumphosphide, zinc oxide, or gallium oxide as a material is used, forexample. A semiconductor substrate in which an insulator region isprovided in the above semiconductor substrate, e.g., a silicon oninsulator (SOI) substrate or the like is used. As the conductorsubstrate, a graphite substrate, a metal substrate, an alloy substrate,a conductive resin substrate, or the like is used. A substrate includinga metal nitride, a substrate including a metal oxide, or the like isused. An insulator substrate provided with a conductor or asemiconductor, a semiconductor substrate provided with a conductor or aninsulator, a conductor substrate provided with a semiconductor or aninsulator, or the like is used. Alternatively, any of these substratesover which an element is provided may be used. As the element providedover the substrate, a capacitor, a resistor, a switching element, alight-emitting element, a memory element, or the like is used.

Alternatively, a flexible substrate may be used as the substrate 400. Asa method for providing the transistor over a flexible substrate, thereis a method in which the transistor is formed over a non-flexiblesubstrate and then the transistor is separated and transferred to thesubstrate 400 which is a flexible substrate. In that case, a separationlayer is preferably provided between the non-flexible substrate and thetransistor. As the substrate 400, a sheet, a film, or a foil containinga fiber may be used. The substrate 400 may have elasticity. Thesubstrate 400 may have a property of returning to its original shapewhen bending or pulling is stopped. Alternatively, the substrate 400 mayhave a property of not returning to its original shape. The substrate400 has a region with a thickness of, for example, greater than or equalto 5 μm and less than or equal to 700 μm, preferably greater than orequal to 10 μm and less than or equal to 500 μm, more preferably greaterthan or equal to 15 μm and less than or equal to 300 μm. When thesubstrate 400 has a small thickness, the weight of the semiconductordevice including the transistor can be reduced. When the substrate 400has a small thickness, even in the case of using glass or the like, thesubstrate 400 may have elasticity or a property of returning to itsoriginal shape when bending or pulling is stopped. Therefore, an impactapplied to the semiconductor device over the substrate 400, which iscaused by dropping or the like, can be reduced. That is, a durablesemiconductor device can be provided.

For the substrate 400 which is a flexible substrate, metal, an alloy,resin, glass, or fiber thereof can be used, for example. The flexiblesubstrate 400 preferably has a lower coefficient of linear expansionbecause deformation due to an environment is suppressed. The flexiblesubstrate 400 is formed using, for example, a material whose coefficientof linear expansion is lower than or equal to 1×10⁻³/K, lower than orequal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, and acrylic. In particular, aramid ispreferably used for the flexible substrate 400 because of its lowcoefficient of linear expansion.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 3 <Method 1 for Manufacturing Transistor>

A method for manufacturing the transistor of the present invention inFIGS. 1A to 1C will be described below with reference to FIGS. 15A to15C, FIGS. 16A to 16C, FIGS. 17A to 17C, FIGS. 18A to 18C, FIGS. 19A to19C, FIGS. 20A to 20C, FIGS. 21A to 21C, and FIGS. 22A to 22C.

First, the substrate 400 is prepared.

Then, the insulator 402 is formed. The insulator 402 may be formed by asputtering method, a chemical vapor deposition (CVD) method, a molecularbeam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, anatomic layer deposition (ALD) method, or the like.

Note that CVD methods can be classified into a plasma enhanced CVD(PECVD) method using plasma, a thermal CVD (TCVD) method using heat, aphoto CVD method using light, and the like. Moreover, the CVD method caninclude a metal CVD (MCVD) method and a metal organic CVD (MOCVD) methoddepending on a source gas.

In the case of a PECVD method, a high quality film can be obtained atrelatively low temperature. Furthermore, a TCVD method does not useplasma and thus causes less plasma damage to an object. For example, awiring, an electrode, an element (e.g., transistor or capacitor), or thelike included in a semiconductor device might be charged up by receivingelectric charges from plasma. In that case, accumulated electric chargesmight break the wiring, electrode, element, or the like included in thesemiconductor device. Such plasma damage is not caused in the case ofusing a TCVD method, and thus the yield of a semiconductor device can beincreased. In addition, since plasma damage does not occur in thedeposition by a TCVD method, a film with few defects can be obtained.

An ALD method also causes less plasma damage to an object. An ALD methoddoes not cause plasma damage during deposition, so that a film with fewdefects can be obtained.

Unlike in a deposition method in which particles ejected from a targetor the like are deposited, in a CVD method and an ALD method, a film isformed by reaction at a surface of an object. Thus, a CVD method and anALD method enable favorable step coverage almost regardless of the shapeof an object. In particular, an ALD method enables excellent stepcoverage and excellent thickness uniformity and can be favorably usedfor covering a surface of an opening with a high aspect ratio, forexample. On the other hand, an ALD method has a relatively lowdeposition rate; thus, it is sometimes preferable to combine an ALDmethod with another deposition method with a high deposition rate suchas a CVD method.

When a CVD method or an ALD method is used, composition of a film to beformed can be controlled with a flow rate ratio of the source gases. Forexample, by the CVD method or the ALD method, a film with a desiredcomposition can be formed by adjusting the flow ratio of a source gas.Moreover, with a CVD method or an ALD method, by changing the flow rateratio of the source gases while forming the film, a film whosecomposition is continuously changed can be formed. In the case where thefilm is formed while changing the flow rate ratio of the source gases,as compared to the case where the film is formed using a plurality ofdeposition chambers, time taken for the deposition can be reducedbecause time taken for transfer and pressure adjustment is omitted.Thus, semiconductor devices can be manufactured with improvedproductivity.

Next, treatment to add oxygen to the insulator 402 may be performed.Examples of the treatment for adding oxygen to the insulator 402 includeion implantation and plasma treatment. Note that oxygen added to theinsulator 402 is excess oxygen.

Next, an insulator to be the insulator 406 a is deposited over theinsulator 402. The insulator to be the insulator 406 a can be depositedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like.

Next, treatment to add oxygen to the insulator to be the insulator 406 amay be performed. Examples of the treatment for adding oxygen to theinsulator to be the insulator 406 a include ion implantation and plasmatreatment. Note that oxygen added to the insulator to be the insulator406 a is excess oxygen. Oxygen is preferably added to a layercorresponding to the insulator to be the insulator 406 a. Next, thesemiconductor to be the semiconductor 406 b is formed over the insulatorto be the insulator 406 a.

Next, first heat treatment is preferably performed. The first heattreatment can be performed at a temperature higher than or equal to 250°C. and lower than or equal to 650° C., preferably higher than or equalto 450° C. and lower than or equal to 600° C., further preferably higherthan or equal to 520° C. and lower than or equal to 570° C. The firstheat treatment is performed in an inert gas atmosphere or an atmospherecontaining an oxidizing gas at 10 ppm or more, 1% or more, or 10% ormore. The first heat treatment may be performed under a reducedpressure. Alternatively, the first heat treatment may be performed insuch a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or10% or more in order to compensate desorbed oxygen. By the first heattreatment, crystallinity of the semiconductor can be increased andimpurities such as hydrogen and moisture can be removed, for example.Alternatively, in the first heat treatment, plasma treatment usingoxygen may be performed under a reduced pressure. The plasma treatmentcontaining oxygen is preferably performed using an apparatus including apower source for generating high-density plasma using microwaves, forexample. Alternatively, a power source for applying a radio frequency(RF) to a substrate side may be provided. The use of high-density plasmaenables high-density oxygen radicals to be produced, and application ofthe RF to the substrate side allows oxygen radicals generated by thehigh-density plasma to be efficiently introduced into the semiconductor406 b. Alternatively, after plasma treatment using an inert gas with theapparatus, plasma treatment using oxygen in order to compensate releasedoxygen may be performed.

Next, a conductor to be a conductor 415 is formed over the semiconductorto be the semiconductor 406 b. The conductor can be formed by asputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. Then, the insulator to be the insulator 406 a, thesemiconductor to be the semiconductor 406 b, and the conductor to be theconductor 415 are processed by a lithography method or the like to forma multilayer film including the insulator 406 a, the semiconductor 406b, and the conductor 415. Here, a top surface of the semiconductor to bethe semiconductor 406 b is damaged when the conductor is formed, wherebythe region 407 is formed. The region 407 includes a region in which theresistance of the semiconductor 406 b is reduced; thus, contactresistance between the conductor 415 and the semiconductor 406 b isreduced. Note that when the multilayer film is formed, the insulator 402is also subjected etching to have a thinned region in some cases. Thatis, the insulator 402 may have a protruding portion in a region incontact with the multilayer film (see FIGS. 15A to 15C).

In the lithography method, first, a resist is exposed to light through aphotomask. Next, a region exposed to light is removed or left using adeveloping solution, so that a resist mask is formed. Then, etchingthrough the resist mask is conducted. As a result, the conductor, thesemiconductor, the insulator, or the like can be processed into adesired shape. The resist mask is formed by, for example, exposure ofthe resist to light using KrF excimer laser light, ArF excimer laserlight, extreme ultraviolet (EUV) light, or the like. Alternatively, aliquid immersion technique may be employed in which a portion between asubstrate and a projection lens is filled with liquid (e.g., water) toperform light exposure. An electron beam or an ion beam may be usedinstead of the above-mentioned light. Note that a photomask is notnecessary in the case of using an electron beam or an ion beam. Notethat dry etching treatment such as ashing or wet etching treatment canbe used for removal of the resist mask. Alternatively, wet etchingtreatment is performed after dry etching treatment. Furtheralternatively, dry etching treatment is performed after wet etchingtreatment.

As a dry etching apparatus, a capacitively coupled plasma (CCP) etchingapparatus including parallel plate type electrodes can be used. Thecapacitively coupled plasma etching apparatus including the parallelplate type electrodes may have a structure in which a high-frequencypower source is applied to one of the parallel plate type electrodes.Alternatively, the capacitively coupled plasma etching apparatus mayhave a structure in which different high-frequency power sources areapplied to one of the parallel plate type electrodes. Alternatively, thecapacitively coupled plasma etching apparatus may have a structure inwhich high-frequency power sources with the same frequency are appliedto the parallel plate type electrodes. Alternatively, the capacitivelycoupled plasma etching apparatus may have a structure in whichhigh-frequency power sources with different frequencies are applied tothe parallel plate type electrodes. Alternatively, a dry etchingapparatus including a high-density plasma source can be used. As the dryetching apparatus including a high-density plasma source, an inductivelycoupled plasma (ICP) etching apparatus can be used, for example.

Next, an insulator 410 a is formed. The insulator 410 a can be formed bya sputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. Alternatively, the insulator 410 a can be formed bya spin coating method, a dipping method, a droplet discharging method(such as an ink-jet method), a printing method (such as screen printingor offset printing), a doctor knife method, a roll coater method, acurtain coater method, or the like.

The insulator 410 a may be formed to have a flat top surface. Forexample, the top surface of the insulator 410 a may have flatnessimmediately after the film formation. Alternatively, for example, theinsulator 410 a may have flatness by removing the insulator and the likefrom the top surface after the film formation so that the top surface ofthe insulator 410 a becomes parallel to a reference surface such as arear surface of the substrate. Such treatment is referred to asplanarization treatment. As the planarization treatment, for example,chemical mechanical polishing (CMP) treatment, dry etching treatment, orthe like can be performed. However, the top surface of the insulator 410a is not necessarily flat.

Next, a resist mask 411 is formed over the insulator 410 a by alithography method or the like. Here, in order to improve the adhesionbetween the top surface of the insulator 410 a and the resist mask 411,for example, an organic coating film may be provided between theinsulator 410 a and the resist mask 411 (see FIGS. 16A to 16C).

Next, first processing is performed until the opening in the insulator410 a reaches a top surface of the conductor 415 by dry etching or thelike. As a gas for the dry etching in the first processing, for example,a C₄F₆ gas, a CF₄ gas, a SF₆ gas, a CHF₃ gas, or the like can be used.Alternatively, an oxygen gas, a helium gas, an argon gas, a hydrogengas, or the like can be added to any of the above gases as appropriate.Here, a C₄F₆ gas to which an oxygen gas is added is preferably used. Asa dry etching apparatus, any of the above-described dry etchingapparatuses can be used; however, a dry etching apparatus in whichhigh-frequency power sources with different frequencies are connected toparallel-plate electrodes is preferably used (see FIGS. 17A to 17C).

Next, the conductor 415 is subjected to second processing by dry etchingor the like so as to be separated into the conductor 416 a 1 and theconductor 416 a 2. As a gas for the dry etching in the secondprocessing, for example, a C₄F₆ gas, a CF₄ gas, a SF₆ gas, a Cl₂ gas, aBCl₃ gas, a SiCl₄ gas, and the like can be used alone or in combination.Alternatively, an oxygen gas, a helium gas, an argon gas, or a hydrogengas can be added to any of the above gases as appropriate. Here, acombination of a CF₄ gas, a Cl₂ gas, and an oxygen gas is preferablyused. As a dry etching apparatus, the above-described dry etchingapparatuses for the first processing may be used (see FIGS. 18A to 18C).

At this time, the semiconductor 406 b has a region that is exposed. Theexposed region of the semiconductor 406 b, which is the region 407, isremoved in some cases. In the second processing, side surfaces of thesemiconductor 406 b are covered with the insulator 410 and thus are notexposed to a gas used for the second processing. That is, etching is notperformed on the side surfaces of the semiconductor 406 b in the secondprocessing; thus, residual components of the gas used for the secondprocessing is prevented from attaching to the surface of thesemiconductor 406 b. Thus, the side surfaces of the semiconductor 406 bare preferably protected.

Next, third processing is performed until the opening in an insulator tobe the insulator 410 reaches the insulator 402 by dry etching or thelike, whereby the insulator 410 is formed. As a dry etching gas used inthe third processing, a C₄F₆ gas to which an oxygen gas is added can beused as in the first processing; however, in the third processing, forexample, the flow rate of the oxygen gas to be added is preferablyincreased. By increasing the flow rate of the oxygen gas compared tothat in the first processing, the etching rate of the resist mask 411 inthe horizontal direction along a surface parallel to the rear surfacethe substrate is made higher than that in the first processing, and thusthe etching rate of the insulator to be the insulator 410 in thehorizontal direction along the surface parallel to the rear surface thesubstrate is made higher than that in the first processing. Thus, asdenoted by dotted lines in FIGS. 19B and 19C, the opening in theinsulator 410 can be formed so as to expand in the horizontal directionalong the surface parallel to the rear surface the substrate (see FIGS.19A to 19C).

A dry etching apparatus that is similar to that used in the firstprocessing and the second processing can be used in the thirdprocessing, for example. In the case where a dry etching apparatus inwhich high-frequency power sources with different frequencies areapplied to the parallel plate type electrodes is used, the electricpower of the high-frequency power source that is applied to one or bothof the parallel plate type electrodes is preferably increased comparedto the case in the first processing. As a result, the etching rate ofthe resist mask 411 in the horizontal direction along the surfaceparallel to the rear surface of the substrate can be made higher thanthat in the first processing, and the same effect as that obtained byincreasing the flow rate of the oxygen gas added as described above canbe obtained. Thus, the opening of the insulator 410 can be formed so asto expand in the horizontal direction along the surface parallel to therear surface of the substrate.

The same dry etching apparatus as that in the first processing and thesecond processing is used in the third processing; thus, the first tothird processing can be successively performed without exposure to air.Therefore, contamination due to attachment of an atmospheric componentto a substrate, corrosion of the insulator, the semiconductor, and theconductor due to reaction between the etching gas remaining on thesubstrate and the atmospheric component, and the like can be prevented.Alternatively, improvement in productivity can be expected when thefirst processing, the second processing, and the third processing aresuccessively performed.

When the first processing and the second processing are each performedby dry etching, an impurity such as the residual components of theetching gas is attached to the exposed region of the semiconductor 406 bin some cases. For example, when a chlorine-based gas is used as anetching gas, chlorine and the like are attached in some cases.Furthermore, when a hydrocarbon-based gas is used as an etching gas,carbon, hydrogen, and the like are attached in some cases. When thesubstrate is exposed to air after the second processing, the exposedregion of the semiconductor 406 b and the like corrode in some cases.Thus, plasma treatment using an oxygen gas is preferably performedsuccessively after the second processing because the impurity can beremoved and corrosion of the exposed region of the semiconductor 406 band the like can be prevented.

Alternatively, the impurity may be reduced by cleaning treatment usingdiluted hydrofluoric acid or the like or cleaning treatment using ozoneor the like, for example. Note that different types of cleaningtreatment may be combined. In such a manner, the exposed region of thesemiconductor 406 b, i.e., a channel formation region has highresistance.

Meanwhile, in the region 407 where the conductors 416 a 1 and 416 a 2and the top surface of the semiconductor 406 b overlap with each other,a value of contact resistance between the conductors 416 a 1 and 416 a 2and the semiconductor 406 b is decreased; thus, favorable transistorcharacteristics can be obtained.

Next, an insulator 406 c_1 is formed, and an insulator 412_1 is formedover the insulator 406 c_1. The insulator 406 c_1 and the insulator412_1 can be deposited by a sputtering method, a CVD method, an MBEmethod, a PLD method, an ALD method, or the like. The insulator 406 c_1and the insulator 412_1 are preferably formed to have the uniformthickness along bottom and side surfaces of an opening formed in theinsulator 410 and the conductors 416 a 1 and 416 a 2. Therefore, the ALDmethod is preferably used.

Next, a conductor 404_1 is formed. The conductor 404_1 can be formed bya sputtering method, a CVD method, an MBE method, a PLD method, an ALDmethod, or the like. The conductor 404_1 is formed so as to fill theopening formed by the insulator 410 and the like. Therefore, the CVDmethod (the MCVD method, in particular) is preferred. A multilayer filmof a conductor deposited by an ALD method or the like and a conductordeposited by a CVD method is preferred in some cases to increaseadhesion of the conductor deposited by an MCVD method. For example, amultilayer film in which titanium nitride and tungsten are formed inthis order or the like may be used (see FIGS. 20A to 20C).

Next, the conductor 404_1, the insulator 412_1, and the insulator 406c_1 are polished and flattened so as to reach the top surface of theinsulator 410 by CMP or the like from the top surface of the conductor404_1, whereby the conductor 404, the insulator 412, and the insulator406 c are formed. Accordingly, the conductor 404 serving as the gateelectrode can be formed in a self-aligned manner without using alithography method. The conductor 404 serving as the gate electrode canbe formed without considering alignment accuracy of the conductor 404serving as the gate electrode and the conductors 416 a 1 and 416 a 2serving as the source electrode and the drain electrode; as a result,the area of the semiconductor device can be reduced. Furthermore,because the lithography process is not necessary, improvement inproductivity due to simplification of the process is expected (see FIGS.21A to 21C).

An insulator to be the insulator 408 can be formed by a sputteringmethod, a CVD method, a MBE method, a PLD method, an ALD method, or thelike. Aluminum oxide is preferably formed as the insulator to be theinsulator 408 using plasma containing oxygen, so that oxygen in theplasma can be added to the top surface of the insulator 410, a region ofthe insulator 412 in contact with the insulator 408, and a region of theinsulator 406 c in contact with the insulator 408 as excess oxygen.Here, a mixed region 414 containing a large amount of excess oxygenmight be formed in the interface between the insulator 408 and theinsulator 410 and the vicinity of the interface. FIGS. 22B and 22Cillustrate the state where the excess oxygen is added to the vicinity ofthe mixed region 414 by arrows.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 408. By the second heat treatment,the excess oxygen contained in the insulator 410 and the mixed region414 is moved to the semiconductor 406 b through the insulators 402and/or 406 a. Furthermore, the excess oxygen contained in the insulator410 and the mixed region 414 is moved to the semiconductor 406 b throughthe insulator 412 and/or the insulator 406 c. Since excess oxygen ismoved to the semiconductor 406 b by passing two paths as describedabove, defects (oxygen vacancies) in the semiconductor 406 b can bereduced (see FIGS. 22B and 22C).

Note that the second heat treatment may be performed at a temperaturesuch that excess oxygen (oxygen) in the insulator 410 and the mixedregion 414 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thesecond heat treatment. Alternatively, the temperature of the second heattreatment is preferably lower than that of the first heat treatment. Thesecond heat treatment is preferably performed at a temperature lowerthan that of the first heat treatment by higher than or equal to 20° C.and lower than or equal to 150° C., preferably higher than or equal to40° C. and lower than or equal to 100° C. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 402 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, an opening reaching the conductor 416 a 1 and an opening reachingthe conductor 416 a 2 are formed in the insulator to be the insulator408 and the insulator 410, and the conductors 431 and 429 are embeddedin their respective openings. Next, an opening reaching the conductor404 is formed in the insulator to be the insulator 408, and theconductor 437 is embedded in the opening, whereby the insulator 408 isformed. Conductors to be the conductors 432, 430, and 438 are formedover the insulator 408 and the conductors 431, 429, and 437, and theconductors 432, 430, and 438 are formed by a lithography method or thelike. Note that formation of the openings reaching the conductors 416 a1 and 416 a 2 in the insulator to be the insulator 408 and the insulator410, and formation of the opening reaching the conductor 404 in theinsulator 408 may be performed at the same time. When the openings areformed at the same time, improvement in productivity due tosimplification of the process is expected.

Through the above steps, the transistor illustrated in FIGS. 1A to 1Ccan be manufactured.

<Method 2 for Manufacturing Transistor>

A method for manufacturing a transistor in FIGS. 5A to 5C having astructure different from that of the transistor in FIGS. 1A to 1C willbe described below with reference to FIG. 3 , FIGS. 23A to 23C, FIGS.24A to 24C, FIGS. 25A to 25C, and FIGS. 26A to 26C. The process up tothe second processing is similar to that in the method 1 formanufacturing a transistor (see FIGS. 18A and 18B).

Next, third processing is performed until the insulator to be theinsulator 410 reaches the insulator 402 by dry etching or the like,whereby the insulator 410 is formed. As a dry etching gas used in thethird processing, a C₄F₆ gas to which an oxygen gas is added, which isthe same as that used in the first processing is used; however, in thethird processing, for example, the flow rate of the oxygen gas to beadded may be increased. By increasing the flow rate of the oxygen gascompared to that in the first processing, the etching rate of the resistmask 411 in the horizontal direction along the surface parallel to therear surface of substrate is made higher than that in the firstprocessing, and thus the etching rate of the insulator to be theinsulator 410 in the horizontal direction along the surface parallel tothe rear surface of the substrate is made higher than that in the firstprocessing. Furthermore, when a gas containing one or more kinds ofgases for etching the conductors 416 a 1 and 416 a 2, e.g., a SF₆ gas, aCF₄ gas, and a Cl₂ gas, is added to a C₄F₆ gas and an oxygen gas, asdenoted by dotted lines in FIGS. 23B and 23C, the opening of theinsulator 410 can be formed so as to expand in the horizontal directionalong the surface parallel to the rear surface of the substrate, andregions can be formed in which the side surfaces of the conductors 416 a1 and 416 a 2 have a tapered angle 446 and a tapered angle 447,respectively. Although not shown in the figures, only top end portionsof the conductors 416 a 1 and 416 a 2 may have tapered cross sections,or the top end portions of the conductors 416 a 1 and 416 a 2 may haveround cross sections.

Accordingly, coverage by the insulator 406 c formed over the sidesurfaces of the conductors 416 a 1 and 416 a 2 and the insulator 412serving as the gate insulator is improved. When coverage by theinsulator 406 c and the insulator 412 is improved, leakage currentflowing through the conductor 404 serving as the gate electrode andthrough the conductors 416 a 1 and 416 a 2 serving as the sourceelectrode and the drain electrode can be kept low. Alternatively,leakage current flowing between the conductor 404 serving as the gateelectrode and the semiconductor 406 b including a channel formationregion can be kept low.

The taper angles 446 and 447 are each greater than or equal to 10° andless than or equal to 90°, preferably, greater than or equal to 30° andless than or equal to 80°. The taper angle 446 can be controlled bychanging an etching rate ratio between an etching rate of the conductor416 a 1 and an etching rate of the insulator 410 in the horizontaldirection along the surface parallel to rear surface of the substrate.The taper angle 447 can be controlled by changing an etching rate ratiobetween an etching rate of the conductor 416 a 2 and an etching rate ofthe insulator 410 in the horizontal direction along the surface parallelto the rear surface of the substrate. For example, when the etching rateratio is 1, the taper angle 446 and the taper angle 447 are each 45°(see FIG. 3 ).

A dry etching apparatus that is similar to that used in the firstprocessing can be used in the third processing, for example.

The same dry etching apparatus as that in the first processing and thesecond processing is used in the third processing; thus, the first tothird processing can be successively performed without exposure to air.Therefore, contamination due to attachment of an atmospheric componentto a substrate, corrosion of the insulator, the semiconductor, and theconductor due to reaction between the etching gas remaining on thesubstrate and the atmospheric component, and the like can be prevented.Alternatively, improvement in productivity can be expected when thefirst processing, the second processing, and the third processing aresuccessively performed.

When the first processing and the second processing are each performedby dry etching, an impurity such as the residual components of theetching gas is attached to the exposed region of the semiconductor 406 bin some cases. For example, when a chlorine-based gas is used as anetching gas, chlorine and the like are attached in some cases.Furthermore, when a hydrocarbon-based gas is used as an etching gas,carbon, hydrogen, and the like are attached in some cases. When thesubstrate is exposed to air after the second processing, the exposedregion of the semiconductor 406 b and the like corrode in some cases.Thus, plasma treatment using an oxygen gas is preferably performedsuccessively after the second processing because the impurity can beremoved and corrosion of the exposed region of the semiconductor 406 band the like can be prevented.

Alternatively, the impurity may be reduced by cleaning treatment usingdiluted hydrofluoric acid or the like or cleaning treatment using ozoneor the like, for example. Note that different types of cleaningtreatment may be combined. In such a manner, the exposed region of thesemiconductor 406 b, i.e., a channel formation region has highresistance.

Meanwhile, in the region 407 where the conductors 416 a 1 and 416 a 2and the top surface of the semiconductor 406 b overlap with each other,a value of contact resistance between the conductors 416 a 1 and 416 a 2and the semiconductor 406 b is decreased; thus, favorable transistorcharacteristics can be obtained.

Next, the insulator 406 c_1 is formed, and the insulator 412_1 is formedover the insulator 406 c_1. The insulator 406 c_1 and the insulator412_1 can be deposited by a sputtering method, a CVD method, an MBEmethod, a PLD method, an ALD method, or the like. The insulator 406 c_1and the insulator 412_1 are preferably formed to have the uniformthickness along the bottom and side surfaces of the opening formed bythe insulator 410 and the conductors 416 a 1 and 416 a 2. Therefore, theALD method is preferably used.

Next, the conductor 404_1 is formed. The conductor 404_1 can be formedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like. The conductor 404_1 is formed so as to fill theopening formed by the insulator 410 and the like. Therefore, the CVDmethod (the MCVD method, in particular) is preferred. A multilayer filmof a conductor deposited by an ALD method or the like and a conductordeposited by a CVD method is preferred in some cases to increaseadhesion of the conductor deposited by an MCVD method. For example, amultilayer film in which titanium nitride and tungsten are formed inthis order or the like may be used (see FIGS. 24A to 24C).

Next, the conductor 404_1, the insulator 412_1, and the insulator 406c_1 are polished and flattened so as to reach the top surface of theinsulator 410 by CMP or the like from the top surface of the conductor404_1, whereby the conductor 404, the insulator 412, and the insulator406 c are formed. Accordingly, the conductor 404 serving as the gateelectrode can be formed in a self-aligned manner without using alithography method. The conductor 404 serving as the gate electrode canbe formed without considering alignment accuracy of the conductor 404serving as the gate electrode and the conductors 416 a 1 and 416 a 2serving as the source electrode and the drain electrode; as a result,the area of the semiconductor device can be reduced. Furthermore,because the lithography process is not necessary, improvement inproductivity due to simplification of the process is expected (see FIGS.25A to 25C).

The insulator to be the insulator 408 can be formed by a sputteringmethod, a CVD method, a MBE method, a PLD method, an ALD method, or thelike. Aluminum oxide is preferably formed as the insulator to be theinsulator 408 using plasma containing oxygen, so that oxygen in theplasma can be added to the top surface of the insulator 410, a region ofthe insulator 412 in contact with the insulator 408, and a region of theinsulator 406 c in contact with the insulator 408 as excess oxygen.Here, a mixed region 414 containing a large amount of excess oxygenmight be formed in the interface between the insulator 408 and theinsulator 410 and the vicinity of the interface. FIGS. 26B and 26Cillustrate the state where the excess oxygen is added to the vicinity ofthe mixed region 414 by arrows.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 408. By the second heat treatment,the excess oxygen contained in the insulator 410 and the mixed region414 is moved to the semiconductor 406 b through the insulators 402and/or 406 a. Furthermore, the excess oxygen contained in the insulator410 and the mixed region 414 is moved to the semiconductor 406 b throughthe insulator 412 and/or the insulator 406 c. Since excess oxygen ismoved to the semiconductor 406 b by passing two paths as describedabove, defects (oxygen vacancies) in the semiconductor 406 b can bereduced (see FIGS. 26A and 26B).

Note that the second heat treatment may be performed at a temperaturesuch that excess oxygen (oxygen) in the insulator 410 and the mixedregion 414 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thesecond heat treatment. Alternatively, the temperature of the second heattreatment is preferably lower than that of the first heat treatment. Thesecond heat treatment is preferably performed at a temperature lowerthan that of the first heat treatment by higher than or equal to 20° C.and lower than or equal to 150° C., preferably higher than or equal to40° C. and lower than or equal to 100° C. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 402 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, an opening reaching the conductor 416 a 1 and an opening reachingthe conductor 416 a 2 are formed in the insulator to be the insulator408 and the insulator 410, and the conductors 431 and 429 are embeddedin their respective openings. Next, an opening reaching the conductor404 is formed in the insulator to be the insulator 408, and theconductor 437 is embedded in the opening, whereby the insulator 408 isformed. Conductors to be the conductors 432, 430, and 438 are formedover the insulator 408 and the conductors 431, 429, and 437, and theconductors 432, 430, and 438 are formed by a lithography method or thelike. Note that formation of the openings reaching the conductors 416 a1 and 416 a 2 in the insulator to be the insulator 408 and the insulator410, and formation of the opening reaching the conductor 404 in theinsulator 408 may be performed at the same time. When the openings areformed at the same time, improvement in productivity due tosimplification of the process is expected.

Through the above steps, the transistor illustrated in FIGS. 5A to 5Ccan be manufactured.

<Method 3 for Manufacturing Transistor>

A method for manufacturing a transistor in FIGS. 1A to 1C which isdifferent from Method 1 for manufacturing a transistor is described withreference to FIGS. 27A to 27C, FIGS. 28A to 28C, FIGS. 29A to 29C, FIGS.30A to 30C, FIGS. 31A to 31C, FIGS. 32A to 32C, and FIGS. 33A to 33C.The process up to the formation of the multilayer film including theinsulator 406 a, the semiconductor 406 b, and the conductor 415 issimilar to that in the Method 1 for manufacturing a transistor andMethod 2 for manufacturing a transistor (see FIGS. 15A and 15B).

Next, the insulator 410 a is formed. The insulator 410 a can be formedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like. Alternatively, the insulator 410 a can beformed by a spin coating method, a dipping method, a droplet dischargingmethod (such as an ink-jet method), a printing method (such as screenprinting or offset printing), a doctor knife method, a roll coatermethod, a curtain coater method, or the like.

The insulator 410 a may be formed to have a flat top surface. Forexample, the top surface of the insulator 410 a may have flatnessimmediately after the film formation. Alternatively, for example, thetop surface of the insulator 410 a may have flatness by removing theupper portion of the insulator and the like after the film formation sothat the top surface of the insulator 410 a becomes parallel to areference surface such as a rear surface of the substrate. Suchtreatment is referred to as planarization treatment. As theplanarization treatment, for example, chemical mechanical polishing(CMP) treatment, dry etching treatment, or the like can be performed.However, the top surface of the insulator 410 a is not necessarily flat.

Next, the resist mask 411 is formed over the insulator 410 a by alithography method or the like. Here, an organic coating film may beprovided between the insulator 410 a and the resist mask 411 in order toimprove the adhesion between the top surface of the insulator 410 a andthe resist mask 411. An opening in the resist mask 411 is formed so asto be larger than that formed by Method 1 for manufacturing a transistorand Method 2 for manufacturing a transistor (see FIGS. 27A, 27B, and27C).

Next, first processing is performed until the opening in the insulator410 a reaches the top surface of the conductor 415 and the top surfaceof the insulator 402 by dry etching or the like, whereby the insulator410 is formed. As a gas for the dry etching in the first processing, forexample, a C₄F₆ gas, a CF₄ gas, a SF₆ gas, a CHF₃ gas, or the like canbe used. Alternatively, an oxygen gas, a helium gas, an argon gas, ahydrogen gas, or the like can be added to any of the above gases asappropriate. Here, a C₄F₆ gas to which an oxygen gas is added ispreferably used. As a dry etching apparatus, any of the above-describeddry etching apparatuses can be used; however, a dry etching apparatus inwhich high-frequency power sources with different frequencies areconnected to the parallel-plate electrodes is preferably used (see FIGS.28A to 28C).

Next, plasma is generated using a gas containing carbon and halogen witha dry etching apparatus and an organic substance 413 is deposited on theresist mask 411, the semiconductor 406 b, and the side surfaces of theinsulator 410 by plasma. Examples of the gas to be used include CHF₃,and C₄F₈. The dry etching apparatus may be the same as that used in thefirst processing. Alternatively, a dry etching apparatus using ahigh-density plasma source may be used; for example, aninductively-coupled plasma etching apparatus can be used (see FIGS. 29Ato 29C). Although the above is the film formation method of an organicfilm using a dry etching apparatus, the present invention is not limitedto this, and an apparatus different from the dry etching apparatus maybe used. Alternatively, an inorganic film may be formed.

Next, the conductor 415 is subjected to the second processing by dryetching or the like so as to be separated into the conductor 416 a 1 andthe conductor 416 a 2. Since the organic substance 413 is formed on theside surfaces of the insulator 410, the conductor 415 is etched alongthe side surfaces of the organic substance 413 on the side surfaces ofthe insulator 410, and as shown in FIG. 30B, the side surfaces of the416 a 1 and 416 a 2 are positioned inward from the side surfaces of theinsulator 410. As a gas for the dry etching in the second processing,for example, a C₄F₆ gas, a CF₄ gas, a SF₆ gas, a Cl₂ gas, a BCl₃ gas, aSiCl₄ gas, and the like can be used alone or in combination.Alternatively, an oxygen gas, a helium gas, an argon gas, or a hydrogengas can be added to any of the above gases as appropriate. Here, acombination of a CF₄ gas, a Cl₂ gas, and an oxygen gas is preferablyused. As a dry etching apparatus, the above-described dry etchingapparatuses for the first processing may be used (see FIGS. 30A to 30C).

As a dry etching apparatus used in the second processing, a dry etchingapparatus that is similar to that used in the first processing can beused.

The deposition of the organic substance 413 and the second processingare performed using the same dry etching apparatus as that in the firstprocessing; thus, the first processing, the deposition of the organicsubstance 413, and the second processing can be successively performedwithout exposure to air. Therefore, contamination due to attachment ofan atmospheric component to a substrate, corrosion of the insulator, thesemiconductor, and the conductor due to reaction between the etching gasremaining on the substrate and the atmospheric component, and the likecan be prevented. Alternatively, improvement in productivity can beexpected by successively performing the first processing, the depositionof the organic substance 413, and the second processing.

When the first processing, the deposition of the organic substance 413,and the second processing are each performed by dry etching, an impuritysuch as the residual components of the etching gas is attached to theexposed region of the semiconductor 406 b in some cases. For example,when a chlorine-based gas is used as an etching gas, chlorine and thelike are attached in some cases. Furthermore, when a hydrocarbon-basedgas is used as an etching gas, carbon, hydrogen, and the like areattached in some cases. When the substrate is exposed to air after thesecond processing, the exposed region of the semiconductor 406 b and thelike corrode in some cases. Thus, plasma treatment using an oxygen gasis preferably performed successively after the second processing becausethe impurity can be removed and corrosion of the exposed region of thesemiconductor 406 b and the like can be prevented. Furthermore, theorganic substance 413 on the side surfaces of the insulator 410 can beremoved by the plasma treatment using an oxygen gas.

Alternatively, the impurity may be reduced by cleaning treatment usingdiluted hydrofluoric acid or the like or cleaning treatment using ozoneor the like, for example. Note that different types of cleaningtreatment may be combined. In such a manner, the exposed region of thesemiconductor 406 b, i.e., a channel formation region has highresistance.

Meanwhile, in the region 407 where the conductors 416 a 1 and 416 a 2and the top surface of the semiconductor 406 b overlap with each other,a value of contact resistance between the conductors 416 a 1 and 416 a 2and the semiconductor 406 b is decreased; thus, favorable transistorcharacteristics can be obtained.

Next, the insulator 406 c_1 is formed, and the insulator 412_1 is formedover the insulator 406 c_1. The insulator 406 c_1 and the insulator412_1 can be deposited by a sputtering method, a CVD method, an MBEmethod, a PLD method, an ALD method, or the like. The insulator 406 c_1and the insulator 412_1 are preferably formed to have the uniformthickness along the bottom and side surfaces of the opening formed bythe insulator 410 and the conductors 416 a 1 and 416 a 2. Therefore, theALD method is preferably used.

Next, the conductor 404_1 is formed. The conductor 404_1 can be formedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like. The conductor 404_1 is formed so as to fill theopening formed by the insulator 410 and the like. Therefore, the CVDmethod (the MCVD method, in particular) is preferred. A multilayer filmof a conductor deposited by an ALD method or the like and a conductordeposited by a CVD method is preferred in some cases to increaseadhesion of the conductor deposited by an MCVD method. For example, amultilayer film in which titanium nitride and tungsten are formed inthis order or the like may be used (see FIGS. 31A to 31C).

Next, the conductor 404_1, the insulator 412_1, and the insulator 406c_1 are polished and flattened so as to reach the top surface of theinsulator 410 by CMP or the like from the top surface of the conductor404_1, whereby the conductor 404, the insulator 412, and the insulator406 c are formed. Accordingly, the conductor 404 serving as the gateelectrode can be formed in a self-aligned manner without using alithography method. The conductor 404 serving as the gate electrode canbe formed without considering alignment accuracy of the conductor 404serving as the gate electrode and the conductors 416 a 1 and 416 a 2serving as the source electrode and the drain electrode; as a result,the area of the semiconductor device can be reduced. Furthermore,because the lithography process is not necessary, improvement inproductivity due to simplification of the process is expected (see FIGS.32A to 32C).

The insulator to be the insulator 408 can be formed by a sputteringmethod, a CVD method, a MBE method, a PLD method, an ALD method, or thelike. Aluminum oxide is preferably formed as the insulator to be theinsulator 408 using plasma containing oxygen, so that oxygen in theplasma can be added to the top surface of the insulator 410, the regionof the insulator 412 in contact with the insulator 408, and the regionof the insulator 406 c in contact with the insulator 408 as excessoxygen. Here, the mixed region 414 containing a large amount of excessoxygen might be formed in the interface between the insulator 408 andthe insulator 410 and the vicinity of the interface. FIGS. 33B and 33Cillustrate the state where the excess oxygen is added to the vicinity ofthe mixed region 414 by arrows.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 408. By the second heat treatment,the excess oxygen contained in the insulator 410 and the mixed region414 is moved to the semiconductor 406 b through the insulators 402and/or 406 a. Furthermore, the excess oxygen contained in the insulator410 and the mixed region 414 is moved to the semiconductor 406 b throughthe insulator 412 and/or the insulator 406 c. Since excess oxygen ismoved to the semiconductor 406 b by passing two paths as describedabove, defects (oxygen vacancies) in the semiconductor 406 b can bereduced (see FIGS. 33A and 33B).

Note that the second heat treatment may be performed at a temperaturesuch that excess oxygen (oxygen) in the insulator 410 and the mixedregion 414 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thesecond heat treatment. Alternatively, the temperature of the second heattreatment is preferably lower than that of the first heat treatment. Thesecond heat treatment is preferably performed at a temperature lowerthan that of the first heat treatment by higher than or equal to 20° C.and lower than or equal to 150° C., preferably higher than or equal to40° C. and lower than or equal to 100° C. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 402 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, the opening reaching the conductor 416 a 1 and the openingreaching the conductor 416 a 2 are formed in the insulator to be theinsulator 408 and the insulator 410, and the conductors 431 and 429 areembedded in their respective openings. Next, the opening reaching theconductor 404 is formed in the insulator to be the insulator 408, andthe conductor 437 is embedded in the opening, whereby the insulator 408is formed. The conductors to be the conductors 432, 430, and 438 areformed over the insulator 408 and the conductors 431, 429, and 437, andthe conductors 432, 430, and 438 are formed by a lithography method orthe like. Note that formation of the openings reaching the conductors416 a 1 and 416 a 2 in the insulator to be the insulator 408 and theinsulator 410, and formation of the opening reaching the conductor 404in the insulator 408 may be performed at the same time. When theopenings are formed at the same time, improvement in productivity due tosimplification of the process is expected.

Through the above steps, the transistor in FIGS. 1A to 1C can bemanufactured by a method different from the Method 1 for manufacturing atransistor.

<Method 4 for Manufacturing Transistor>

A method for manufacturing the transistor in FIGS. 5A to 5C which isdifferent from Method 2 for manufacturing a transistor is described withreference to FIG. 3 , FIGS. 34A to 34C, FIGS. 35A to 35C, FIGS. 36A to36C, and FIGS. 37A to 37C. The process up to the deposition of theorganic substance 413 on the resist mask 411, the semiconductor 406 b,and the side surfaces of the insulator 410 is similar to that in Method3 for manufacturing a transistor (see FIGS. 29A and 29B).

Next, the conductor 415 is subjected to the second processing by dryetching or the like, so as to be separated into the conductor 416 a 1and the conductor 416 a 2. As a gas for the dry etching in the secondprocessing, for example, a C₄F₆ gas, a CF₄ gas, a SF₆ gas, a Cl₂ gas, aBCl₃ gas, a SiCl₄ gas, or the like can be used alone or in combination.Alternatively, an oxygen gas, a helium gas, an argon gas, a hydrogengas, or the like can be added to any of the above gases as appropriate.Here, the flow rate of the oxygen gas may be increased compared to thatin the etching gas used in the second processing in Method 3 formanufacturing a transistor. By increasing the flow rate of the oxygengas, the etching rate of the organic film deposited on the side surfacesof the insulator 410 in the horizontal direction along the surfaceparallel to the rear surface of the substrate is made higher than thatin the second processing by Method 3 for manufacturing a transistor, andthus, as denoted by dotted lines in FIGS. 34B and 34C, the organicsubstance 413 on the side surfaces of the insulator 410 is etched in thehorizontal direction along the surface parallel to the rear surfacesubstrate and the regions can be formed in which the side surfaces ofthe conductors 416 a 1 and 416 a 2 have the tapered angle 446 and thetapered angle 447, respectively. Although not shown in the figures, onlytop end portions of the conductors 416 a 1 and 416 a 2 may have crosssections with tapered shape, or the top end portions of the conductors416 a 1 and 416 a 2 may have round cross sections.

Accordingly, coverage by the insulator 406 c formed over the sidesurfaces of the conductors 416 a 1 and 416 a 2 and the insulator 412serving as the gate insulator is improved. When coverage by theinsulator 406 c and the insulator 412 is improved, leakage currentflowing through the conductor 404 serving as the gate electrode andthrough the conductors 416 a 1 and 416 a 2 serving as the sourceelectrode and the drain electrode can be kept low. Alternatively,leakage current flowing between the conductor 404 serving as the gateelectrode and the semiconductor 406 b including the channel formationregion can be kept low.

The taper angles 446 and 447 are each greater than or equal to 10° andless than or equal to 90°, preferably, greater than or equal to 30° andless than or equal to 80°. The taper angle 446 can be controlled bychanging an etching rate ratio between the etching rate of the conductor416 a 1 and an etching rate of the organic substance 413 deposited onthe insulator 410 in the horizontal direction along the surface parallelto the rear surface of the substrate. The taper angle 447 can becontrolled by changing an etching rate ratio between the etching rate ofthe conductor 416 a 2 and an etching rate of the organic substance 413deposited on the insulator 410 in the horizontal direction along thesurface parallel to the rear surface of the substrate. For example, whenthe etching rate ratio is 1, the taper angle 446 and the taper angle 447are each 45° (see FIG. 3 )

As a dry etching apparatus used in the second processing, a dry etchingapparatus that is similar to that used in the first processing can beused.

The deposition of the organic substance 413 and the second processingare performed using the same dry etching apparatus as that in the firstprocessing; thus, the first processing, the deposition of the organicsubstance 413, and the second processing can be successively performedwithout exposure to air. Therefore, contamination due to attachment ofan atmospheric component to a substrate, corrosion of the insulator, thesemiconductor, and the conductor due to reaction between the etching gasremaining on the substrate and the atmospheric component, and the likecan be prevented. Alternatively, improvement in productivity can beexpected by successively performing the first processing, the secondprocessing, and the third processing.

When the first processing, the deposition of the organic substance 413,and the second processing are each performed by dry etching, an impuritysuch as the residual components of the etching gas may be attached tothe exposed region of the semiconductor 406 b. For example, when achlorine-based gas is used as an etching gas, chlorine and the like maybe attached. Furthermore, when a hydrocarbon-based gas is used as anetching gas, carbon, hydrogen, and the like may be attached. When thesubstrate is exposed to air after the second processing, the exposedregion of the semiconductor 406 b and the like may corrode. Thus, plasmatreatment using an oxygen gas that is successively performed after thesecond processing is preferably performed because the impurity can beremoved and corrosion of the exposed region of the semiconductor 406 band the like can be prevented. Furthermore, the organic substance 413 onthe side surfaces of the insulator 410 can be removed by the plasmatreatment using an oxygen gas.

Alternatively, the impurity may be reduced by cleaning treatment usingdiluted hydrofluoric acid or the like or cleaning treatment using ozoneor the like, for example. Note that different types of cleaningtreatment may be combined. In such a manner, the exposed region of thesemiconductor 406 b, i.e., a channel formation region has highresistance.

Meanwhile, in the region 407 where the conductors 416 a 1 and 416 a 2and the top surface of the semiconductor 406 b overlap with each other,a value of contact resistance between the conductors 416 a 1 and 416 a 2and the semiconductor 406 b is decreased; thus, favorable transistorcharacteristics can be obtained.

Next, the insulator 406 c_1 is formed, and the insulator 412_1 is formedover the insulator 406 c_1. The insulator 406 c_1 and the insulator412_1 can be deposited by a sputtering method, a CVD method, an MBEmethod, a PLD method, an ALD method, or the like. The insulator 406 c_1and the insulator 412_1 are preferably formed to have the uniformthickness along the bottom and side surfaces of the opening formed bythe insulator 410 and the conductors 416 a 1 and 416 a 2. Therefore, theALD method is preferably used.

Next, the conductor 404_1 is formed. The conductor 404_1 can be formedby a sputtering method, a CVD method, an MBE method, a PLD method, anALD method, or the like. The conductor 404_1 is formed so as to fill theopening formed by the insulator 410 and the like. Therefore, the CVDmethod (the MCVD method, in particular) is preferred. A multilayer filmof a conductor deposited by an ALD method or the like and a conductordeposited by a CVD method is preferred in some cases to increaseadhesion of the conductor deposited by an MCVD method. For example, amultilayer film in which titanium nitride and tungsten are formed inthis order or the like may be used (see FIGS. 35A to 35C).

Next, the conductor 404_1, the insulator 412_1, and the insulator 406c_1 are polished and flattened so that the opening reaches the topsurface of the insulator 410 by CMP or the like from the top surface ofthe conductor 404_1, whereby the conductor 404, the insulator 412, andthe insulator 406 c are formed. Accordingly, the conductor 404 servingas the gate electrode can be formed in a self-aligned manner withoutusing a lithography method. The conductor 404 serving as the gateelectrode can be formed without considering alignment accuracy of theconductor 404 serving as the gate electrode and the conductors 416 a 1and 416 a 2 serving as the source electrode and the drain electrode; asa result, the area of the semiconductor device can be reduced.Furthermore, because the lithography process is not necessary,improvement in productivity due to simplification of the process isexpected (see FIGS. 36A to 36C).

The insulator to be the insulator 408 can be formed by a sputteringmethod, a CVD method, a MBE method, a PLD method, an ALD method, or thelike. Aluminum oxide is preferably formed as the insulator to be theinsulator 408 using plasma containing oxygen, so that oxygen in theplasma can be added to the top surface of the insulator 410, the regionof the insulator 412 in contact with the insulator 408, and the regionof the insulator 406 c in contact with the insulator 408 as excessoxygen. Here, the mixed region 414 containing a large amount of excessoxygen might be formed in the interface between the insulator 408 andthe insulator 410 and the vicinity of the interface. FIGS. 37B and 37Cillustrate the state where the excess oxygen is added to the vicinity ofthe mixed region 414 by arrows.

Second heat treatment may be performed at any time after the formationof the insulator to be the insulator 408. By the second heat treatment,the excess oxygen contained in the insulator 410 and the mixed region414 is moved to the semiconductor 406 b through the insulators 402and/or 406 a. Furthermore, the excess oxygen contained in the insulator410 and the mixed region 414 is moved to the semiconductor 406 b throughthe insulator 412 and/or the insulator 406 c. Since excess oxygen ismoved to the semiconductor 406 b by passing two paths as describedabove, defects (oxygen vacancies) in the semiconductor 406 b can bereduced (see FIGS. 37A and 37B).

Note that the second heat treatment may be performed at a temperaturesuch that excess oxygen (oxygen) in the insulator 410 and the mixedregion 414 is diffused to the semiconductor 406 b. For example, thedescription of the first heat treatment may be referred to for thesecond heat treatment. Alternatively, the temperature of the second heattreatment is preferably lower than that of the first heat treatment. Thesecond heat treatment is preferably performed at a temperature lowerthan that of the first heat treatment by higher than or equal to 20° C.and lower than or equal to 150° C., preferably higher than or equal to40° C. and lower than or equal to 100° C. Accordingly, superfluousrelease of excess oxygen (oxygen) from the insulator 402 can beinhibited. Note that the second heat treatment is not necessarilyperformed when heating during formation of the films can work as heattreatment comparable to the second heat treatment.

Next, the opening reaching the conductor 416 a 1 and the openingreaching the conductor 416 a 2 are formed in the insulator to be theinsulator 408 and the insulator 410, and the conductors 431 and 429 areembedded in their respective openings. Next, the opening reaching theconductor 404 is formed in the insulator to be the insulator 408, andthe conductor 437 is embedded in the opening, whereby the insulator 408is formed. The conductors to be the conductors 432, 430, and 438 areformed over the insulator 408 and the conductors 431, 429, and 437, andthe conductors 432, 430, and 438 are formed by a lithography method orthe like. Note that formation of the openings reaching the conductors416 a 1 and 416 a 2 in the insulator to be the insulator 408 and theinsulator 410, and formation of the opening reaching the conductor 404in the insulator 408 may be performed at the same time. When theopenings are formed at the same time, improvement in productivity due tosimplification of the process is expected.

Through the above steps, the transistor in FIGS. 5A to 5C can bemanufactured by a method different from the Method 2 for manufacturing atransistor. At least part of this embodiment can be implemented incombination with any of the embodiments described in this specificationas appropriate.

Embodiment 4 <Memory Device 1>

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 38A and 38B.

The semiconductor device illustrated in FIG. 38A includes a transistor3200 using a first semiconductor, a transistor 3300 using a secondsemiconductor, and a capacitor 3400. Note that any of theabove-described transistors can be used as the transistor 3300.

Note that the transistor 3300 is preferably a transistor with a lowoff-state current. For example, a transistor using an oxidesemiconductor can be used as the transistor 3300. Since the off-statecurrent of the transistor 3300 is low, stored data can be retained for along period at a predetermined node of the semiconductor device. Inother words, power consumption of the semiconductor device can bereduced because refresh operation becomes unnecessary or the frequencyof refresh operation can be extremely low.

In FIG. 38A, a first wiring 3001 is electrically connected to a sourceof the transistor 3200. A second wiring 3002 is electrically connectedto a drain of the transistor 3200. A third wiring 3003 is electricallyconnected to one of the source and the drain of the transistor 3300. Afourth wiring 3004 is electrically connected to the gate of thetransistor 3300. The gate of the transistor 3200 and the other of thesource and the drain of the transistor 3300 are electrically connectedto one electrode of the capacitor 3400. A fifth wiring 3005 iselectrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 38A has a feature that the potential ofthe gate of the transistor 3200 can be retained, and thus enableswriting, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 ison, so that the transistor 3300 is turned on. Accordingly, the potentialof the third wiring 3003 is supplied to a node FG where the gate of thetransistor 3200 and the one electrode of the capacitor 3400 areelectrically connected to each other. That is, a predetermined electriccharge is supplied to the gate of the transistor 3200 (writing). Here,one of two kinds of electric charges providing different potentiallevels (hereinafter referred to as a low-level electric charge and ahigh-level electric charge) is supplied. After that, the potential ofthe fourth wiring 3004 is set to a potential at which the transistor3300 is off, so that the transistor 3300 is turned off. Thus, theelectric charge is held at the node FG (retaining).

Since the off-state current of the transistor 3300 is low, the electriccharge of the node FG is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of electric charge retained in the node FG. This is because inthe case of using an n-channel transistor as the transistor 3200, anapparent threshold voltage V_(th_H) at the time when the high-levelelectric charge is given to the gate of the transistor 3200 is lowerthan an apparent threshold voltage V_(th_L) at the time when thelow-level electric charge is given to the gate of the transistor 3200.Here, an apparent threshold voltage refers to the potential of the fifthwiring 3005 which is needed to make the transistor 3200 be in “onstate.” Thus, the potential of the fifth wiring 3005 is set to apotential V₀ which is between V_(th_H) and V_(th_L), whereby electriccharge supplied to the node FG can be determined. For example, in thecase where the high-level electric charge is supplied to the node FG inwriting and the potential of the fifth wiring 3005 is V₀ (>V_(th_H)),the transistor 3200 is brought into “on state.” In the case where thelow-level electric charge is supplied to the node FG in writing, evenwhen the potential of the fifth wiring 3005 is V₀ (<V_(th_L)), thetransistor 3200 still remains in “off state.” Thus, the data retained inthe node FG can be read by determining the potential of the secondwiring 3002.

Note that in the case where memory cells are arrayed, it is necessarythat data of a desired memory cell be read in read operation. The fifthwiring 3005 of memory cells from which data is not read may be suppliedwith a potential at which the transistor 3200 is turned off regardlessof the electric charge supplied to the node FG, that is, a potentiallower than V_(th_H), whereby only data of a desired memory cell can beread. Alternatively, the fifth wiring 3005 of the memory cells fromwhich data is not read may be supplied with a potential at which thetransistor 3200 is brought into “on state” regardless of the electriccharge supplied to the node FG, that is, a potential higher thanV_(th_L), whereby only data of a desired memory cell can be read.

<Structure 1 of Semiconductor Device>

FIG. 39 is a cross-sectional view of the semiconductor device in FIG.38A. The semiconductor device shown in FIG. 39 includes the transistor3200, the transistor 3300, and the capacitor 3400. The transistor 3300and the capacitor 3400 are provided over the transistor 3200. Althoughan example where the transistor illustrated in FIGS. 1A to 1C is used asthe transistor 3300 is shown, a semiconductor device of one embodimentof the present invention is not limited thereto. The description of theabove transistor is referred to.

The transistor 3200 illustrated in FIG. 39 is a transistor using asemiconductor substrate 450. The transistor 3200 includes a region 474 ain the semiconductor substrate 450, a region 474 b in the semiconductorsubstrate 450, an insulator 462, and a conductor 454.

In the transistor 3200, the regions 474 a and 474 b have a function as asource region and a drain region. The insulator 462 has a function as agate insulator. The conductor 454 has a function as a gate electrode.Therefore, resistance of a channel formation region can be controlled bya potential applied to the conductor 454. In other words, conduction ornon-conduction between the region 474 a and the region 474 b can becontrolled by the potential applied to the conductor 454.

For the semiconductor substrate 450, a single-material semiconductorsubstrate of silicon, germanium, or the like or a compound semiconductorsubstrate of silicon carbide, silicon germanium, gallium arsenide,indium phosphide, zinc oxide, gallium oxide, or the like may be used,for example. A single crystal silicon substrate is preferably used asthe semiconductor substrate 450.

For the semiconductor substrate 450, a semiconductor substrate includingimpurities imparting n-type conductivity is used. However, asemiconductor substrate including impurities imparting p-typeconductivity may be used as the semiconductor substrate 450. In thatcase, a well including impurities imparting the n-type conductivity maybe provided in a region where the transistor 3200 is formed.Alternatively, the semiconductor substrate 450 may be an i-typesemiconductor substrate.

The top surface of the semiconductor substrate 450 preferably has a(110) plane. Thus, on-state characteristics of the transistor 3200 canbe improved.

The regions 474 a and 474 b are regions including impurities impartingthe p-type conductivity. Accordingly, the transistor 3200 has astructure of a p-channel transistor.

Note that although the transistor 3200 is illustrated as a p-channeltransistor, the transistor 3200 may be an n-channel transistor.

Note that the transistor 3200 is separated from an adjacent transistorby the region 460 and the like. The region 460 is an insulating region.

The semiconductor illustrated in FIG. 39 includes an insulator 464, aninsulator 466, an insulator 468, an insulator 470, an insulator 472, aninsulator 475, the insulator 402, the insulator 410, the insulator 408,an insulator 428, an insulator 465, an insulator 467, an insulator 469,an insulator 498, a conductor 480 a, a conductor 480 b, a conductor 480c, a conductor 478 a, a conductor 478 b, a conductor 478 c, a conductor476 a, a conductor 476 b, a conductor 476 c, a conductor 479 a, aconductor 479 b, a conductor 479 c, a conductor 477 a, a conductor 477b, a conductor 477 c, a conductor 484 a, a conductor 484 b, a conductor484 c, a conductor 484 d, a conductor 483 a, a conductor 483 b, aconductor 483 c, a conductor 483 d, a conductor 483 e, a conductor 483f, a conductor 485 a, a conductor 485 b, a conductor 485 c, a conductor485 d, a conductor 487 a, a conductor 487 b, a conductor 487 c, aconductor 488 a, a conductor 488 b, a conductor 488 c, a conductor 490a, a conductor 490 b, a conductor 489 a, a conductor 489 b, a conductor491 a, a conductor 491 b, a conductor 491 c, a conductor 492 a, aconductor 492 b, a conductor 492 c, a conductor 494, a conductor 496, aninsulator 406 a, a semiconductor 406 b, and an insulator 406 c.

The insulator 464 is provided over the transistor 3200. The insulator466 is over the insulator 464. The insulator 468 is over the insulator466. The insulator 470 is placed over the insulator 468. The insulator472 is placed over the insulator 470. The insulator 475 is placed overthe insulator 472. The transistor 3300 is provided over the insulator475. The insulator 408 is provided over the transistor 3300. Theinsulator 428 is provided over the insulator 408. The insulator 465 isover the insulator 428. The capacitor 3400 is provided over theinsulator 465. The insulator 469 is provided over the capacitor 3400.

The insulator 464 includes an opening reaching the region 474 a, anopening reaching the region 474 b, and an opening reaching the conductor454, in which the conductor 480 a, the conductor 480 b, and theconductor 480 c are embedded, respectively.

In addition, the insulator 466 includes an opening reaching theconductor 480 a, an opening reaching the conductor 480 b, and an openingreaching the conductor 480 c, in which the conductor 478 a, theconductor 478 b, and the conductor 478 c are embedded, respectively.

In addition, the insulator 468 includes an opening reaching theconductor 478 a, an opening reaching the conductor 478 b, and an openingreaching the conductor 478 c, in which the conductor 476 a, theconductor 476 b, and the conductor 476 c are embedded, respectively.

The conductor 479 a in contact with the conductor 476 a, the conductor479 b in contact with the conductor 476 b, and the conductor 479 c incontact with the conductor 476 c are over the insulator 468. Theinsulator 472 includes an opening reaching the conductor 479 a throughthe insulator 470, an opening reaching the conductor 479 b through theinsulator 470, and an opening reaching the conductor 479 c through theinsulator 470. In the openings, the conductor 477 a, the conductor 477b, or the conductor 477 c is embedded.

The insulator 475 includes an opening overlapping with the channelformation region of the transistor 3300, an opening reaching theconductor 477 a, an opening reaching the conductor 477 b, and an openingreaching the conductor 477 c. In the openings, the conductor 484 a, theconductor 484 b, the conductor 484 c, and the conductor 484 d isembedded.

The conductor 484 d may have a function as a bottom gate electrode ofthe transistor 3300. Alternatively, for example, electriccharacteristics such as the threshold voltage of the transistor 3300 maybe controlled by application of a constant potential to the conductor484 d. Further alternatively, for example, the conductor 484 d and thetop gate electrode of the transistor 3300 may be electrically connectedto each other. Thus, the on-state current of the transistor 3300 can beincreased. A punch-through phenomenon can be suppressed; thus, stableelectric characteristics in the saturation region of the transistor 3300can be obtained.

In addition, the insulator 402 includes an opening reaching theconductor 484 a, an opening reaching the conductor 484 c, and an openingreaching the conductor 484 b.

The insulator 428 includes two openings reaching the conductor 484 a andthe conductor 484 b through the insulator 408, the insulator 410, andthe insulator 402, two openings reaching a conductor of one of thesource electrode and the drain electrode of the transistor 3300 throughthe insulator 408 and the insulator 410, and an opening reaching aconductor of the gate electrode of the transistor 3300 through theinsulator 408. In the openings, the conductor 483 a, the conductor 483c, the conductor 483 e, the conductor 483 f, and the conductor 483 d areembedded.

The conductor 485 a in contact with the conductors 483 a and 483 e, theconductor 485 b in contact with the conductor 483 b, the conductor 485 cin contact with the conductor 483 c and the conductor 483 f, and theconductor 485 d in contact with the conductor 483 d are over theinsulator 428. The insulator 465 has an opening reaching the conductor485 a, an opening reaching the conductor 485 b, and an opening reachingthe conductor 485 c. In the openings, the conductor 487 a, the conductor487 b, and the conductor 487 c are embedded.

The conductor 488 a in contact with the conductor 487 a, the conductor488 b in contact with the conductor 487 b, and the conductor 488 c incontact with the conductor 487 c are over the insulator 465. Inaddition, the insulator 467 includes an opening reaching the conductor488 a and an opening reaching the conductor 488 b. In the openings, theconductor 490 a and the conductor 490 b are embedded. The conductor 488c is in contact with the conductor 494 which is one of the electrodes ofthe capacitor 3400.

The conductor 489 a in contact with the conductor 490 a and theconductor 489 b in contact with the conductor 490 b are over theinsulator 467. The insulator 469 includes an opening reaching theconductor 489 a, an opening reaching the conductor 489 b, an openingreaching the conductor 496 which is the other of electrodes of thecapacitor 3400. In the openings, the conductor 491 a, the conductor 491b, and the conductor 491 c are embedded.

The conductor 492 a in contact with the conductor 491 a, the conductor492 b in contact with the conductor 491 b, and the conductor 492 c incontact with the conductor 491 c are over the insulator 469.

The insulators 464, 466, 468, 470, 472, 475, 402, 410, 408, 428, 465,467, 469, and 498 may each be formed to have, for example, asingle-layer structure or a stacked-layer structure including aninsulator containing boron, carbon, nitrogen, oxygen, fluorine,magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium,germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, ortantalum. The insulator 401 may be formed of, for example, aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, gallium oxide, germanium oxide, yttriumoxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide,or tantalum oxide.

The insulator that has a function of blocking oxygen and impurities suchas hydrogen is preferably included in at least one of the insulators464, 466, 468, 470, 472, 475, 402, 410, 408, 428, 465, 467, 469, and498. When an insulator that has a function of blocking oxygen andimpurities such as hydrogen is placed near the transistor 3300, theelectric characteristics of the transistor 3300 can be stable.

An insulator with a function of blocking oxygen and impurities such ashydrogen may have a single-layer structure or a stacked-layer structureincluding an insulator containing, for example, boron, carbon, nitrogen,oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine,argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium,hafnium, or tantalum may be used.

Each of the conductors 480 a, 480 b, 480 c, 478 a, 478 b, 478 c, 476 a,476 b, 476 c, 479 a, 479 b, 479 c, 477 a, 477 b, 477 c, 484 a, 484 b,484 c, 484 d, 483 a, 483 b, 483 c, 483 d, 483 e, 483 f, 485 a, 485 b,485 c, 485 d, 487 a, 487 b, 487 c, 488 a, 488 b, 488 c, 490 a, 490 b,489 a, 489 b, 491 a, 491 b, 491 c, 492 a, 492 b, 492 c, 494, and 496 mayhave a single-layer structure or a stacked-layer structure including aconductor containing, for example, one or more kinds of boron, nitrogen,oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium,manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. Analloy or a compound may be used, for example, and a conductor containingaluminum, a conductor containing copper and titanium, a conductorcontaining copper and manganese, a conductor containing indium, tin, andoxygen, a conductor containing titanium and nitrogen, or the like may beused.

An oxide semiconductor is preferably used as the semiconductor 406 b.However, silicon (including strained silicon), germanium, silicongermanium, silicon carbide, gallium arsenide, aluminum gallium arsenide,indium phosphide, gallium nitride, an organic semiconductor, or the likecan be used in some cases.

As the insulator 406 a and the insulator 406 c, oxides containing one ormore, or two or more elements other than oxygen included in thesemiconductor 406 b are preferably used. However, silicon (includingstrained silicon), germanium, silicon germanium, silicon carbide,gallium arsenide, aluminum gallium arsenide, indium phosphide, galliumnitride, an organic semiconductor, or the like can be used in somecases.

The source or drain of the transistor 3200 is electrically connected tothe conductor that is one of the source electrode and the drainelectrode of the transistor 3300 through the conductor 480 a, theconductor 478 a, the conductor 476 a, the conductor 479 a, the conductor477 a, the conductor 484 a, the conductor 483 a, the conductor 485 a,and the conductor 483 e. The conductor 454 that is the gate electrode ofthe transistor 3200 is electrically connected to the conductor that isthe other of the source electrode and the drain electrode of thetransistor 3300 through the conductor 480 c, the conductor 478 c, theconductor 476 c, the conductor 479 c, the conductor 477 c, the conductor484 c, the conductor 483 c, the conductor 485 c, and the conductor 483f.

The capacitor 3400 includes the conductor 494 which is one of theelectrodes of the capacitor 3400 and electrically connected to one ofthe source electrode and the drain electrode of the transistor 3300through the conductor 483 f, the conductor 485 c, the conductor 487 c,and the conductor 488 c, the insulator 498, the conductor 496 which isthe other electrode of the capacitor 3400. The capacitor 3400 ispreferably formed above or below the transistor 3300 because thesemiconductor can be reduced in size.

For the structures of other components, the description of FIGS. 1A to1C and the like can be referred to as appropriate.

A semiconductor device in FIG. 40 is the same as the semiconductordevice in FIG. 39 except the structure of the transistor 3200.Therefore, the description of the semiconductor device in FIG. 39 isreferred to for the semiconductor device in FIG. 40 . Specifically, inthe semiconductor device in FIG. 40 , the transistor 3200 is a FIN-typetransistor. The effective channel width is increased in the FIN-typetransistor 3200, whereby the on-state characteristics of the transistor3200 can be improved. In addition, since contribution of the electricfield of the gate electrode can be increased, the off-statecharacteristics of the transistor 3200 can be improved. Note that thetransistor 3200 may be a p-channel transistor or an n-channeltransistor.

Although an example in which the transistor 3300 is over the transistor3200 and the capacitor 3400 is over the transistor 3300 is illustratedin this embodiment, one or more transistors including a semiconductorsimilar to the transistor 3300 may be provided over the transistor 3200.With such a structure, the degree of integration of the semiconductordevice can be further increased.

<Memory device 2>

The Semiconductor Device in FIG. 38B is Different from the SemiconductorDevice in FIG. 38A in that the transistor 3200 is not provided. Also inthis case, data can be written and retained in a manner similar to thatof the semiconductor device in FIG. 38A.

Reading of data in the semiconductor device in FIG. 38B is described.When the transistor 3300 is turned on, the third wiring 3003 which is ina floating state and the capacitor 3400 are electrically connected toeach other, and the charge is redistributed between the third wiring3003 and the capacitor 3400. As a result, the potential of the thirdwiring 3003 is changed. The amount of change in potential of the thirdwiring 3003 varies depending on the potential of the one of theelectrode of the capacitor 3400 (or the charge accumulated in thecapacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V) (C_(B)+C), where V is the potentialof the one electrode of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of the oneelectrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential ofthe third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potentialV₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor may be usedfor a driver circuit for driving a memory cell, and a transistorincluding the second semiconductor may be stacked over the drivercircuit as the transistor 3300.

When including a transistor using an oxide semiconductor and having anlow off-state current, the semiconductor device described above canretain stored data for a long time. In other words, refresh operationbecomes unnecessary or the frequency of the refresh operation can beextremely low, which leads to a sufficient reduction in powerconsumption. Moreover, stored data can be retained for a long time evenwhen power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device, high voltage is not needed forwriting data and deterioration of elements is less likely to occur.Unlike in a conventional nonvolatile memory, for example, it is notnecessary to inject and extract electrons into and from a floating gate;thus, a problem such as deterioration of an insulator is not caused.That is, the semiconductor device of one embodiment of the presentinvention does not have a limit on the number of times data can berewritten, which is a problem of a conventional nonvolatile memory, andthe reliability thereof is drastically improved. Furthermore, data iswritten depending on the state of the transistor (on or off), wherebyhigh-speed operation can be easily achieved. At least part of thisembodiment can be implemented in combination with any of the embodimentsdescribed in this specification as appropriate.

Embodiment 5 <Structure 2 of Semiconductor Device>

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention is described with reference todrawings.

<Cross-Sectional Structure>

FIGS. 41A and 41B are cross-sectional views of a semiconductor device ofone embodiment of the present invention. In FIG. 41A, X1-X2 directionrepresents a channel length direction, and in FIG. 41B, Y1-Y2 directionrepresents a channel width direction. The semiconductor deviceillustrated in FIGS. 41A and 41B includes a transistor 2200 containing afirst semiconductor material in a lower portion and a transistor 2100containing a second semiconductor material in an upper portion. In FIGS.41A and 41B, an example is illustrated in which the transistorillustrated in FIGS. 1A to 1C is used as the transistor 2100 containingthe second semiconductor material.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor using a material other than an oxidesemiconductor, such as single crystal silicon, can operate at high speedeasily. In contrast, a transistor using an oxide semiconductor anddescribed in the above embodiment as an example has excellentsubthreshold characteristics and a minute structure. Furthermore, thetransistor can operate at a high speed because of its high switchingspeed and has low leakage current because of its low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIGS. 41A and 41B illustrate a structure in which the transistor 2100 isprovided over the transistor 2200 with an insulator 2201, an insulator2207, and an insulator 2208 provided therebetween. A plurality ofwirings 2202 are provided between the transistor 2200 and the transistor2100. Furthermore, wirings and electrodes provided over and under theinsulators are electrically connected to each other through a pluralityof plugs 2203 embedded in the insulators. An insulator 2204 covering thetransistor 2100 and a wiring 2205 over the insulator 2204 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in a lower portion, hydrogen in aninsulator provided in the vicinity of the semiconductor film of thetransistor 2200 terminates dangling bonds of silicon; accordingly, thereliability of the transistor 2200 can be improved. Meanwhile, in thecase where an oxide semiconductor is used for the transistor 2100provided in an upper portion, hydrogen in an insulator provided in thevicinity of the semiconductor film of the transistor 2100 becomes afactor of generating carriers in the oxide semiconductor; thus, thereliability of the transistor 2100 might be decreased. Therefore, in thecase where the transistor 2100 using an oxide semiconductor is providedover the transistor 2200 using a silicon-based semiconductor material,it is particularly effective that the insulator 2207 having a functionof preventing diffusion of hydrogen is provided between the transistors2100 and 2200. The insulator 2207 makes hydrogen remain in the lowerportion, thereby improving the reliability of the transistor 2200. Inaddition, since the insulator 2207 suppresses diffusion of hydrogen fromthe lower portion to the upper portion, the reliability of thetransistor 2100 also can be improved.

The insulator 2207 can be, for example, formed using aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film having a function of preventing diffusionof hydrogen is preferably formed over the transistor 2100 to cover thetransistor 2100 including an oxide semiconductor film. For the blockingfilm, a material that is similar to that of the insulator 2207 can beused, and in particular, an aluminum oxide film is preferably used.Using the aluminum oxide film, excess oxygen can be added to theinsulator under the aluminum oxide film in the deposition, and theexcess oxygen moves to the oxide semiconductor layer of the transistor2100 by heat treatment, which has an effect of repairing a defect in theoxide semiconductor layer. The aluminum oxide film has a high shielding(blocking) effect of preventing penetration of both oxygen andimpurities such as hydrogen and moisture. Thus, by using the aluminumoxide film as the blocking film covering the transistor 2100, release ofoxygen from the oxide semiconductor film included in the transistor 2100and entry of water and hydrogen into the oxide semiconductor film can beprevented. Note that as the block film, the insulator 2204 having astacked-layer structure may be used, or the block film may be providedunder the insulator 2204.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. An example of a cross-sectional view in this case is shown inFIGS. 41E and 41F. An insulator 2212 is provided over a semiconductorsubstrate 2211. The semiconductor substrate 2211 includes a projectingportion with a thin tip (also referred to a fin). Note that an insulatormay be provided over the projecting portion. The insulator functions asa mask for preventing the semiconductor substrate 2211 from being etchedwhen the projecting portion is formed. The projecting portion does notnecessarily have the thin tip; a projecting portion with a cuboid-likeprojecting portion and a projecting portion with a thick tip arepermitted, for example. A gate insulator 2214 is provided over theprojecting portion of the semiconductor substrate 2211, and a gateelectrode 2213 is provided over the gate insulator 2214. Source anddrain regions 2215 are formed in the semiconductor substrate 2211. Notethat here is shown an example in which the semiconductor substrate 2211includes the projecting portion; however, a semiconductor device of oneembodiment of the present invention is not limited thereto. For example,a semiconductor region having a projecting portion may be formed byprocessing an SOI substrate.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 6 [CMOS Circuit]

A circuit diagram in FIG. 41C shows a configuration of a so-called CMOScircuit in which a p-channel transistor 2200 and an n-channel transistor2100 are connected to each other in series and in which gates of themare connected to each other.

[Analog Switch]

A circuit diagram in FIG. 41D shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as a so-called analogswitch. At least part of this embodiment can be implemented incombination with any of the embodiments described in this specificationas appropriate.

Embodiment 7 <CPU>

A CPU including a semiconductor device such as any of theabove-described transistors or the above-described memory device isdescribed below.

FIG. 42 is a block diagram illustrating a configuration example of a CPUincluding any of the above-described transistors as a component.

The CPU illustrated in FIG. 42 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and an ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 42 isjust an example of a simplified structure, and an actual CPU may have avariety of structures depending on the application. For example, the CPUmay have the following configuration: a structure including the CPUillustrated in FIG. 42 or an arithmetic circuit is considered as onecore; a plurality of the cores are included; and the cores operate inparallel. The number of bits that the CPU can process in an internalarithmetic circuit or in a data bus can be 8, 16, 32, or 64, forexample.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 processes an interrupt request from an external input/output deviceor a peripheral circuit depending on its priority or a mask state. Theregister controller 1197 generates an address of the register 1196, andreads/writes data from/to the register 1196 depending on the state ofthe CPU.

In the CPU illustrated in FIG. 42 , a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of theabove-described transistors, the above-described memory device, or thelike can be used.

In the CPU illustrated in FIG. 42 , the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of thepower supply voltage to the memory cell in the register 1196 can bestopped.

FIG. 43 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the above-described memory device can be used as the circuit 1202.When supply of a power supply voltage to the memory element 1200 isstopped, GND (0 V) or a potential at which the transistor 1209 in thecircuit 1202 is turned off continues to be input to a gate of thetransistor 1209. For example, the gate of the transistor 1209 isgrounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the line, or thelike is actively utilized.

A control signal WE is input to a first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 43illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 43 , a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 43 , the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a film formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon film or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor. Further alternatively, in the memory element 1200, atransistor in which a channel is formed in an oxide semiconductor can beincluded besides the transistor 1209, and a transistor in which achannel is formed in a layer including a semiconductor other than anoxide semiconductor or the substrate 1190 can be used for the rest ofthe transistors.

As the circuit 1201 in FIG. 43 , for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By using the above-described memory element 1200 for a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Further, shortly after thesupply of the power supply voltage is restarted, the memory element canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency (RF) tag.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 8 <Imaging Device>

FIG. 44A is a top view illustrating an example of an imaging device 200of one embodiment of the present invention. The imaging device 200includes a pixel portion 210 and peripheral circuits for driving thepixel portion 210 (a peripheral circuit 260, a peripheral circuit 270, aperipheral circuit 280, and a peripheral circuit 290). The pixel portion210 includes a plurality of pixels 211 arranged in a matrix with p rowsand q columns (p and q are each a natural number greater than or equalto 2). The peripheral circuit 260, the peripheral circuit 270, theperipheral circuit 280, and the peripheral circuit 290 are eachconnected to a plurality of pixels 211, and a signal for driving theplurality of pixels 211 is supplied. In this specification and the like,in some cases, “a peripheral circuit” or “a driver circuit” indicatesall of the peripheral circuits 260, 270, 280, and 290. For example, theperipheral circuit 260 can be regarded as part of the peripheralcircuit.

The imaging device 200 preferably includes a light source 291. The lightsource 291 can emit detection light P1.

The peripheral circuit includes at least one of a logic circuit, aswitch, a buffer, an amplifier circuit, and a converter circuit. Theperipheral circuit may be formed over a substrate where the pixelportion 210 is formed. Alternatively, a semiconductor device such as anIC chip may be used as part or the whole of the peripheral circuit. Notethat as the peripheral circuit, one or more of the peripheral circuits260, 270, 280, and 290 may be omitted.

As illustrated in FIG. 44B, the pixels 211 may be provided to beinclined in the pixel portion 210 included in the imaging device 200.When the pixels 211 are obliquely arranged, the distance between pixels(pitch) can be shortened in the row direction and the column direction.Accordingly, the quality of an image taken with the imaging device 200can be improved.

<Configuration Example 1 of Pixel>

The pixel 211 included in the imaging device 200 is formed with aplurality of subpixels 212, and each subpixel 212 is combined with afilter which transmits light with a specific wavelength band (colorfilter), whereby data for achieving color image display can be obtained.

FIG. 45A is a top view showing an example of the pixel 211 with which acolor image is obtained. The pixel 211 illustrated in FIG. 45A includesthe subpixel 212 provided with a color filter that transmits light witha red (R) wavelength band (also referred to as a subpixel 212R), asubpixel 212 provided with a color filter that transmits light with agreen (G) wavelength band (also referred to as a subpixel 212G), and asubpixel 212 provided with a color filter that transmits light with ablue (B) wavelength band (also referred to as a subpixel 212B). Thesubpixel 212 can function as a photosensor.

The subpixel 212 (the subpixel 212R, the subpixel 212G, and the subpixel212B) is electrically connected to a wiring 231, a wiring 247, a wiring248, a wiring 249, and a wiring 250. In addition, the subpixel 212R, thesubpixel 212G, and the subpixel 212B are connected to respective wirings253 which are independent from one another. In this specification andthe like, for example, the wiring 248, the wiring 249, and the wiring250 that are connected to the pixel 211 in the n-th row are referred toas a wiring 248[n], a wiring 249[n], and a wiring 250[n], respectively.For example, the wiring 253 connected to the pixel 211 in the m-thcolumn is referred to as a wiring 253[m]. Note that in FIG. 45A, thewirings 253 connected to the subpixel 212R, the subpixel 212G, and thesubpixel 212B in the pixel 211 in the m-th column are referred to as awiring 253[m]R, a wiring 253[m]G, and a wiring 253[m]B. The subpixels212 are electrically connected to the peripheral circuit through theabove wirings.

The imaging device 200 has a structure in which the subpixel 212 iselectrically connected to the subpixel 212 in an adjacent pixel 211 thatis provided with a color filter that transmits light with the samewavelength band as the subpixel 212, via a switch. FIG. 45B shows aconnection example of the subpixels 212: the subpixel 212 in the pixel211 arranged in an n-th (n is an integer greater than or equal to 1 andless than or equal to p) row and an m-th (m is an integer greater thanor equal to 1 and less than or equal to q) column and the subpixel 212in the adjacent pixel 211 arranged in an (n+1)-th row and the m-thcolumn. In FIG. 45B, the subpixel 212R arranged in the n-th row and them-th column and the subpixel 212R arranged in the (n+1)-th row and them-th column are connected to each other via a switch 201. The subpixel212G arranged in the n-th row and the m-th column and the subpixel 212Garranged in the (n+1)-th row and the m-th column are connected to eachother via a switch 202. The subpixel 212B arranged in the n-th row andthe m-th column and the subpixel 212B arranged in the (n+1)-th row andthe m-th column are connected to each other via a switch 203.

The color filter used in the subpixel 212 is not limited to red (R),green (G), and blue (B) color filters, and color filters that transmitlight of cyan (C), yellow (Y), and magenta (M) may be used. By provisionof the subpixels 212 that sense light with three different wavelengthbands in one pixel 211, a full-color image can be obtained.

The pixel 211 including the subpixel 212 provided with a color filtertransmitting yellow (Y) light may be provided, in addition to thesubpixels 212 provided with the color filters transmitting red (R),green (G), and blue (B) light. The pixel 211 including the subpixel 212provided with a color filter transmitting blue (B) light may beprovided, in addition to the subpixels 212 provided with the colorfilters transmitting cyan (C), yellow (Y), and magenta (M) light. Whenthe subpixels 212 sensing light with four different wavelength bands areprovided in one pixel 211, the reproducibility of colors of an obtainedimage can be increased.

For example, in FIG. 45A, in regard to the subpixel 212 sensing a redwavelength band, the subpixel 212 sensing a green wavelength band, andthe subpixel 212 sensing a blue wavelength band, the pixel number ratio(or the light receiving area ratio) thereof is not necessarily 1:1:1.For example, it is possible to employ the Bayer arrangement, in whichthe ratio of the number of pixels (the ratio of light-receiving areas)is set to red:green:blue=1:2:1. Alternatively, the pixel number ratio(the ratio of light receiving area) of red and green to blue may be1:6:1.

Although the number of subpixels 212 provided in the pixel 211 may beone, two or more subpixels are preferably provided. For example, whentwo or more subpixels 212 sensing the same wavelength band are provided,the redundancy is increased, and the reliability of the imaging device200 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbsor reflects visible light is used as the filter, the imaging device 200that senses infrared light can be achieved.

Furthermore, when a neutral density (ND) filter (dark filter) is used,output saturation which occurs when a large amount of light enters aphotoelectric conversion element (light-receiving element) can beprevented. With a combination of ND filters with different dimmingcapabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 211 may be provided with alens. An arrangement example of the pixel 211, a filter 254, and a lens255 is described with cross-sectional views in FIGS. 46A and 46B. Withthe lens 255, the photoelectric conversion element provided in thesubpixel 212 can receive incident light efficiently. Specifically, asillustrated in FIG. 46A, light 256 enters a photoelectric conversionelement 220 through the lens 255, the filter 254 (a filter 254R, afilter 254G, and a filter 254B), a pixel circuit 230, and the like whichare provided in the pixel 211.

However, part of the light 256 indicated by arrows might be blocked bysome wirings 257 as indicated by a region surrounded with dashed-dottedlines. Thus, a preferable structure is such that the lens 255 and thefilter 254 are provided on the photoelectric conversion element 220 sideas illustrated in FIG. 46B, whereby the photoelectric conversion element220 can efficiently receive the light 256. When the light 256 enters thephotoelectric conversion element 220 from the photoelectric conversionelement 220 side, the imaging device 200 with high sensitivity can beprovided.

As the photoelectric conversion element 220 illustrated in FIGS. 46A and46B, a photoelectric conversion element in which a p-n junction or ap-i-n junction is formed may be used.

The photoelectric conversion element 220 may be formed using a substancethat has a function of absorbing a radiation and generating electriccharges. Examples of the substance that has a function of absorbing aradiation and generating electric charges include selenium, lead iodide,mercury iodide, gallium arsenide, cadmium telluride, and cadmium zincalloy.

For example, when selenium is used for the photoelectric conversionelement 220, the photoelectric conversion element 220 can have a lightabsorption coefficient in a wide wavelength range, such as visiblelight, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 211 included in the imaging device 200 may include thesubpixel 212 with a first filter in addition to the subpixel 212illustrated in FIGS. 46A and 46B.

<Configuration Example 2 of Pixel>

An example of a pixel including a transistor using silicon and atransistor using an oxide semiconductor according to one embodiment ofthe present invention is described below.

FIGS. 47A and 47B are each a cross-sectional view of an element includedin an imaging device.

The imaging device illustrated in FIG. 47A includes a transistor 351including silicon over a silicon substrate 300, transistors 353 and 354which include an oxide semiconductor and are stacked over the transistor351, and a photodiode 360 provided in a silicon substrate 300 andincluding an anode 361 and a cathode 362. The transistors and thephotodiode 360 are electrically connected to various plugs 370 andwirings 371. In addition, an anode 361 of the photodiode 360 iselectrically connected to the plug 370 through a low-resistance region363

The imaging device includes a layer 305 including the transistor 351provided on the silicon substrate 300 and the photodiode 360 provided inthe silicon substrate 300, a layer 320 which is in contact with thelayer 305 and includes the wirings 371, a layer 331 which is in contactwith the layer 320 and includes the transistors 353 and 354, and a layer340 which is in contact with the layer 331 and includes a wiring 372 anda wiring 373.

Note that in the example of cross-sectional view in FIG. 47A, alight-receiving surface of the photodiode 360 is provided on the sideopposite to a surface of the silicon substrate 300 where the transistor351 is formed. With the structure, an optical path can be obtainedwithout the influence by the transistors or wirings, and therefore, apixel with a high aperture ratio can be formed. Thus, a pixel with ahigh aperture ratio can be formed. Note that the light-receiving surfaceof the photodiode 360 can be the same as the surface where thetransistor 351 is formed.

In the case where a pixel is formed with use of only transistors usingan oxide semiconductor, the layer 305 may include the transistor usingan oxide semiconductor. Alternatively, the layer 305 may be omitted, andthe pixel may include only transistors using an oxide semiconductor.

In addition, in the cross-sectional view in FIG. 47A, the photodiode 360in the layer 305 and the transistor in the layer 331 can be formed so asto overlap with each other. Thus, the degree of integration of pixelscan be increased. In other words, the resolution of the imaging devicecan be increased.

An imaging device shown in FIG. 47B includes a photodiode 365 in thelayer 340 and over the transistor. In FIG. 47B, the layer 305 includesthe transistor 351 and a transistor 352 using silicon, the layer 320includes the wiring 371, the layer 331 includes the transistors 353 and354 using an oxide semiconductor layer, the layer 340 includes thephotodiode 365. The photodiode 365 includes a semiconductor layer 366, asemiconductor layer 367, and a semiconductor layer 368, and iselectrically connected to the wiring 373 and a wiring 374 through theplug 370.

The element structure shown in FIG. 47B can increase the aperture ratio.

Alternatively, a PIN diode element formed using an amorphous siliconfilm, a microcrystalline silicon film, or the like may be used as thephotodiode 365. In the photodiode 365, an n-type semiconductor layer368, an i-type semiconductor layer 367, and a p-type semiconductor layer366 are stacked in this order. The i-type semiconductor layer 367 ispreferably formed using amorphous silicon. The p-type semiconductorlayer 366 and the n-type semiconductor layer 368 can each be formedusing amorphous silicon, microcrystalline silicon, or the like whichincludes a dopant imparting the corresponding conductivity type. Aphotodiode in which the photodiode 365 is formed using amorphous siliconhas high sensitivity in a visible light wavelength region, and thereforecan easily sense weak visible light.

Here, an insulator 380 is provided between the layer 305 including thetransistor 351 and the photodiode 360 and the layer 331 including thetransistors 353 and 354. However, there is no limitation on the positionof the insulator 380.

Hydrogen in an insulator provided in the vicinity of a channel formationregion of the transistor 351 terminates dangling bonds of silicon;accordingly, the reliability of the transistor 351 can be improved. Incontrast, hydrogen in the insulator provided in the vicinity of thetransistor 353, the transistor 354, and the like becomes one of factorsgenerating a carrier in the oxide semiconductor. Thus, the hydrogen maycause a reduction of the reliability of the transistor 354, thetransistor 354, and the like. Therefore, in the case where thetransistor using an oxide semiconductor is provided over the transistorusing a silicon-based semiconductor, it is preferable that the insulator380 having a function of blocking hydrogen be provided between thetransistors. When the hydrogen is confined below the insulator 380, thereliability of the transistor 351 can be improved. In addition, thehydrogen can be prevented from being diffused from a part below theinsulator 380 to a part above the insulator 380; thus, the reliabilityof the transistor 353, the transistor 354, and the like can beincreased. It is preferable to form the insulator 381 over thetransistors 353 and 354 because oxygen diffusion can be prevented in theoxide semiconductor.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 9 <RF Tag>

In this embodiment, an RF tag that includes the transistor described inthe above embodiments or the memory device described in the aboveembodiment is described with reference to FIG. 48 .

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have extremely high reliability in order to beused for this purpose.

A configuration of the RF tag will be described with reference to FIG.48 . FIG. 48 is a block diagram illustrating a configuration example ofan RF tag.

As shown in FIG. 48 , an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, a structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be usedas the memory circuit 810. Since the memory circuit of one embodiment ofthe present invention can retain data even when not powered, the memorycircuit can be favorably used for an RF tag. Furthermore, the memorycircuit of one embodiment of the present invention needs power (voltage)needed for data writing significantly lower than that needed in aconventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. Furthermore, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 10 <Display Device>

A display device of one embodiment of the present invention is describedbelow with reference to FIGS. 49A to 49C and FIGS. 50A and 50B.

Examples of a display element provided in the display device include aliquid crystal element (also referred to as a liquid crystal displayelement) and a light-emitting element (also referred to as alight-emitting display element). The light-emitting element includes, inits category, an element whose luminance is controlled by a current orvoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like. Adisplay device including an EL element (EL display device) and a displaydevice including a liquid crystal element (liquid crystal displaydevice) are described below as examples of the display device.

Note that the display device described below includes in its category apanel in which a display element is sealed and a module in which an ICsuch as a controller is mounted on the panel.

The display device described below refers to an image display device ora light source (including a lighting device). The display deviceincludes any of the following modules: a module provided with aconnector such as an FPC or TCP; a module in which a printed wiringboard is provided at the end of TCP; and a module in which an integratedcircuit (IC) is mounted directly on a display element by a COG method.

FIGS. 49A to 49C illustrate an example of an EL display device of oneembodiment of the present invention. FIG. 49A is a circuit diagram of apixel in an EL display device. FIG. 49B is a top view showing the wholeof the EL display device. FIG. 49C is a cross-sectional view taken alongpart of dashed-dotted line M-N in FIG. 49B.

FIG. 49A illustrates an example of a circuit diagram of a pixel used inan EL display device.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. Particularly in the case where the number ofportions to which a terminal is connected might be more than one, it isnot necessary to specify the portions to which the terminal isconnected. Therefore, it might be possible to constitute one embodimentof the invention by specifying only portions to which some of terminalsof an active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

The EL display device illustrated in FIG. 49A includes a switchingelement 743, a transistor 741, a capacitor 742, and a light-emittingelement 719.

Note that FIG. 49A and the like each illustrate an example of a circuitstructure; therefore, a transistor can be provided additionally. Incontrast, for each node in FIG. 49A and the like, it is possible not toprovide an additional transistor, switch, passive element, or the like.

A gate of the transistor 741 is electrically connected to one terminalof the switching element 743 and one electrode of the capacitor 742. Asource of the transistor 741 is electrically connected to the otherelectrode of the capacitor 742 and one electrode of the light-emittingelement 719. A power supply potential VDD is supplied to a drain of thetransistor 741. The other terminal of the switching element 743 iselectrically connected to a signal line 744. A constant potential issupplied to the other electrode of the light-emitting element 719. Theconstant potential is a ground potential GND or a potential lower thanthe ground potential GND.

It is preferable to use a transistor as the switching element 743. Whenthe transistor is used as the switching element, the area of a pixel canbe reduced, so that the EL display device can have high resolution. Asthe switching element 743, a transistor formed through the same step asthe transistor 741 can be used, so that EL display devices can bemanufactured with high productivity. Note that as the transistor 741and/or the switching element 743, the transistor illustrated in FIGS. 1Ato 1C can be used, for example.

FIG. 49B is a top view of the EL display device. The EL display deviceincludes a substrate 700, a substrate 750, a sealant 734, a drivercircuit 735, a driver circuit 736, a pixel 737, and an FPC 732. Thesealant 734 is provided between the substrate 700 and the substrate 750so as to surround the pixel 737, the driver circuit 735, and the drivercircuit 736. Note that the driver circuit 735 and/or the driver circuit736 may be provided outside the sealant 734.

FIG. 49C is a cross-sectional view of the EL display device taken alongpart of dashed-dotted line M-N in FIG. 49B.

FIG. 49C illustrates a structure of the transistor 741 including aninsulator 712 over the substrate 700; a conductor 704 a; an insulator706 a that is over the insulator 712 a and the conductor 704 a andpartly overlaps with the conductor 704 a; a semiconductor 706 b over theinsulator 706 a; conductors 716 a 1 and 716 a 2 in contact with a topsurface of the semiconductor 706 b; an insulator 710 over the conductors716 a 1 and 716 a 2; an insulator 706 c over the semiconductor 706 b; aninsulator 718 b over the insulator 706 c; and a conductor 714 that isover the insulator 718 b and overlaps with the semiconductor 706 b. Notethat the structure of the transistor 741 is just an example; thetransistor 741 may have a structure different from that illustrated inFIG. 49C.

In the transistor 741 illustrated in FIG. 49C, the conductor 704 aserves as a gate electrode, the insulator 712 a serves as a gateinsulator, the conductor 716 a 1 serves as a source electrode, theconductor 716 a 2 serves as a drain electrode, the insulator 718 bserves as a gate insulator, and the conductor 714 a serves as a gateelectrode. Note that in some cases, electric characteristics of theinsulator 706 a, the semiconductor 706 b, and the insulator 706 c changeif light enters the insulator 706 a, the semiconductor 706 b, and theinsulator 706 c. To prevent this, it is preferable that one or more ofthe conductor 704 a, the conductor 716 a 1, the conductor 716 a 2, andthe conductor 714 a have a light-blocking property.

FIG. 49C illustrates a structure of the capacitor 742 including aninsulator 706 d that is over a conductor 704 b provided over thesubstrate 700 and partly overlaps with the conductor 704 b; asemiconductor 706 e over the insulator 706 d; conductors 716 a 3 and 716a 4 in contact with a top surface of the semiconductor 706 e; theinsulator 710 over the conductors 716 a 3 and 716 a 4; an insulator 706f over the semiconductor 706 e; the insulator 718 b over the insulator706 f; and a conductor 714 b that is over the insulator 718 b andoverlaps with the semiconductor 706 e.

In the capacitor 742, the conductor 704 b serves as one electrode andthe conductor 714 b serves as the other electrode.

Thus, the capacitor 742 can be formed using a film of the transistor741. The conductor 704 a and the conductor 704 b are preferablyconductors of the same kind, in which case the conductor 704 a and theconductor 704 b can be formed through the same step. Furthermore, theconductor 714 a and the conductor 714 b are preferably conductors of thesame kind, in which case the conductor 714 a and the conductor 714 b canbe formed through the same step.

The capacitor 742 illustrated in FIG. 49C has a large capacitance perarea occupied by the capacitor. Therefore, the EL display deviceillustrated in FIG. 49C has high display quality. Note that thestructure of the capacitor 742 is just an example and may be differentfrom that illustrated in FIG. 49C.

An insulator 728 is provided over the transistor 741 and the capacitor742, and an insulator 720 is provided over the insulator 728. Here, theinsulator 728 and the insulator 720 may have an opening reaching theconductor 716 a 1 that serves as the source electrode of the transistor741. A conductor 781 is provided over the insulator 720. The conductor781 may be electrically connected to the transistor 741 through theopening in the insulator 728 and the insulator 720.

A partition wall 784 having an opening reaching the conductor 781 isprovided over the conductor 781. A light-emitting layer 782 in contactwith the conductor 781 through the opening provided in the partitionwall 784 is provided over the partition wall 784. A conductor 783 isprovided over the light-emitting layer 782. A region where the conductor781, the light-emitting layer 782, and the conductor 783 overlap withone another serves as the light-emitting element 719. In FIG. 49C, theFPC 732 is connected to the wiring 733 a via a terminal 731. Note thatthe wiring 733 a may be formed using the same kind of conductor as theconductor of the transistor 741 or using the same kind of semiconductoras the semiconductor of the transistor 741.

So far, examples of the EL display device are described. Next, anexample of a liquid crystal display device is described.

FIG. 50A is a circuit diagram showing a structural example of a pixel ofthe liquid crystal display device. A pixel illustrated in FIG. 50Aincludes a transistor 751, a capacitor 752, and an element (liquidcrystal element) 753 in which a space between a pair of electrodes isfilled with a liquid crystal.

One of a source and a drain of the transistor 751 is electricallyconnected to a signal line 755, and a gate of the transistor 751 iselectrically connected to a scan line 754.

One electrode of the capacitor 752 is electrically connected to theother of the source and the drain of the transistor 751, and the otherelectrode of the capacitor 752 is electrically connected to a wiring forsupplying a common potential.

One electrode of the liquid crystal element 753 is electricallyconnected to the other of the source and the drain of the transistor751, and the other electrode of the liquid crystal element 753 iselectrically connected to a wiring to which a common potential issupplied. The common potential supplied to the wiring electricallyconnected to the other electrode of the capacitor 752 may be differentfrom that supplied to the other electrode of the liquid crystal element753.

Note the description of the liquid crystal display device is made on theassumption that the top view of the liquid crystal display device issimilar to that of the EL display device. FIG. 50B is a cross-sectionalview of the liquid crystal display device taken along dashed-dotted lineM-N in FIG. 49B. In FIG. 50B, the FPC 732 is connected to the wiring 733a via the terminal 731. Note that the wiring 733 a may be formed usingthe same kind of conductor as the conductor of the transistor 751 orusing the same kind of semiconductor as the semiconductor of thetransistor 751.

For the transistor 751, the description of the transistor 741 isreferred to. For the capacitor 752, the description of the capacitor 742is referred to. Note that the structure of the capacitor 752 in FIG. 50Bcorresponds to, but is not limited to, the structure of the capacitor742 in FIG. 49C.

Note that in the case where an oxide semiconductor is used as thesemiconductor of the transistor 751, the off-state current of thetransistor 751 can be extremely small. Therefore, an electric chargeheld in the capacitor 752 is unlikely to leak, so that the voltageapplied to the liquid crystal element 753 can be maintained for a longtime. Accordingly, the transistor 751 can be kept off during a period inwhich moving images with few motions or a still image are/is displayed,whereby power for the operation of the transistor 751 can be saved inthat period; accordingly a liquid crystal display device with low powerconsumption can be provided. Furthermore, the area occupied by thecapacitor 752 can be reduced; thus, a liquid crystal display device witha high aperture ratio or a high-resolution liquid crystal display devicecan be provided.

An insulator 721 is provided over the transistor 751 and the capacitor752. The insulator 721 has an opening portion reaching the transistor751. A conductor 791 is provided over the insulator 721. The conductor791 is electrically connected to the transistor 751 through the openingportion in the insulator 721.

An insulator 792 serving as an alignment film is provided over theconductor 791. A liquid crystal layer 793 is provided over the insulator792. An insulator 794 serving as an alignment film is provided over theliquid crystal layer 793. A spacer 795 is provided over the insulator794. A conductor 796 is provided over the spacer 795 and the insulator794. A substrate 797 is provided over the conductor 796.

Owing to the above-described structure, a display device including acapacitor occupying a small area, a display device with high displayquality, or a high-resolution display device can be provided.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ various modes or caninclude various elements. The display element, the display device, thelight-emitting element, or the light-emitting device includes at leastone of an electroluminescence (EL) element (e.g., an EL elementincluding organic and inorganic materials, an organic EL element, or aninorganic EL element), an LED (e.g., a white LED, a red LED, a greenLED, or a blue LED), a transistor (a transistor that emits lightdepending on current), an electron emitter, a liquid crystal element,electronic ink, an electrophoretic element, a grating light valve (GLV),a plasma display panel (PDP), a display element using micro electromechanical systems (MEMS), a digital micromirror device (DMD), a digitalmicro shutter (DMS), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, a display element including a carbon nanotube, and the like.Other than the above, display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electrical ormagnetic effect may be included.

Note that examples of display devices having EL elements include an ELdisplay. Examples of a display device including an electron emitterinclude a field emission display (FED), an SED-type flat panel display(SED: surface-conduction electron-emitter display), and the like.Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of a display devices having electronic ink oran electrophoretic element include electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

Note that in the case of using an LED, graphene or graphite may beprovided under an electrode or a nitride semiconductor of the LED.Graphene or graphite may be a multilayer film in which a plurality oflayers are stacked. As described above, provision of graphene orgraphite enables easy formation of a nitride semiconductor thereover,such as an n-type GaN semiconductor including crystals. Furthermore, ap-type GaN semiconductor including crystals or the like can be providedthereover, and thus the LED can be formed. Note that an AlN layer may beprovided between the n-type GaN semiconductor including crystals andgraphene or graphite. The GaN semiconductors included in the LED may beformed by MOCVD. Note that when the graphene is provided, the GaNsemiconductors included in the LED can also be formed by a sputteringmethod.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 11

In this embodiment, a display module using a semiconductor device of oneembodiment of the present invention is described with reference to FIG.51 .

<Display Module>

In a display module 6000 in FIG. 51 , a touch panel 6004 connected to anFPC 6003, a display panel 6006 connected to an FPC 6005, a backlightunit 6007, a frame 6009, a printed board 6010, and a battery 6011 areprovided between an upper cover 6001 and a lower cover 6002. Note thatthe backlight unit 6007, the battery 6011, the touch panel 6004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 6006 and an integratedcircuit mounted on a printed circuit board.

The shapes and sizes of the upper cover 6001 and the lower cover 6002can be changed as appropriate in accordance with the sizes of the touchpanel 6004 and the display panel 6006.

The touch panel 6004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 6006. Acounter substrate (sealing substrate) of the display panel 6006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 6006 so that an optical touch panel function is added.An electrode for a touch sensor may be provided in each pixel of thedisplay panel 6006 so that a capacitive touch panel function is added.

The backlight unit 6007 includes a light source 6008. The light source6008 may be provided at an end portion of the backlight unit 6007 and alight diffusing plate may be used.

The frame 6009 protects the display panel 6006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated fromthe printed board 6010. The frame 6009 may function as a radiator plate.

The printed board 6010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 6011 provided separatelymay be used. Note that the battery 6011 is not necessary in the casewhere a commercial power source is used.

The display module 6000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 12 <Package Using a Lead Frame Interposer>

FIG. 52A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer. In the package illustratedin FIG. 52A, a chip 551 corresponding to the semiconductor device of oneembodiment of the present invention is connected to a terminal 552 overan interposer 550 by wire bonding. The terminal 552 is placed on asurface of the interposer 550 on which the chip 551 is mounted. The chip551 may be sealed by a mold resin 553, in which case the chip 551 issealed such that part of each of the terminals 552 is exposed.

FIG. 52B illustrates the structure of a module of an electronic device(mobile phone) in which a package is mounted on a circuit board. In themodule of the mobile phone in FIG. 52B, a package 602 and a battery 604are mounted on a printed wiring board 601. The printed wiring board 601is mounted on a panel 600 including a display element by an FPC 603.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

Embodiment 13

In this embodiment, electronic devices and lighting devices of oneembodiment of the present invention will be described with reference todrawings.

<Electronic Device>

Electronic devices and lighting devices can be fabricated using thesemiconductor device of one embodiment of the present invention. Inaddition, highly reliable electronic devices and lighting devices can befabricated using the semiconductor device of one embodiment of thepresent invention. Furthermore, electronic devices and lighting devicesincluding touch sensors with improved detection sensitivity can befabricated using the semiconductor device of one embodiment of thepresent invention.

Examples of electronic devices include a television set (also referredto as a television or a television receiver), a monitor of a computer orthe like, a digital camera, a digital video camera, a digital photoframe, a mobile phone (also referred to as a mobile phone device), aportable game machine, a portable information terminal, an audioreproducing device, a large game machine such as a pinball machine, andthe like.

In the case of having flexibility, the light-emitting device or lightingdevice of one embodiment of the present invention can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

As examples of the secondary battery, a lithium ion secondary batterysuch as a lithium polymer battery (lithium ion polymer battery) using agel electrolyte, a lithium ion battery, a nickel-hydride battery, anickel-cadmium battery, an organic radical battery, a lead-acid battery,an air secondary battery, a nickel-zinc battery, and a silver-zincbattery can be given.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for contactless power transmission.

FIG. 53A illustrates a portable game machine including a housing 7101, ahousing 7102, a display portion 7103, a display portion 7104, amicrophone 7105, speakers 7106, an operation key 7107, a stylus 7108,and the like. The semiconductor device of one embodiment of the presentinvention can be used for an integrated circuit, a CPU, or the likeincorporated in the housing 7101. When the light-emitting deviceaccording of one embodiment of the present invention is used as thedisplay portion 7103 or 7104, it is possible to provide a user-friendlyportable game machine with quality that hardly deteriorates. Althoughthe portable game machine illustrated in FIG. 53A includes two displayportions, the display portion 7103 and the display portion 7104, thenumber of display portions included in the portable game machine is notlimited to two.

FIG. 53B illustrates a smart watch, which includes a housing 7302, adisplay portion 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like. Thesemiconductor device of one embodiment of the present invention can beused for a memory, a CPU, or the like incorporated in the housing 7302.

FIG. 53C illustrates a portable information terminal, which includes adisplay portion 7502 incorporated in a housing 7501, operation buttons7503, an external connection port 7504, a speaker 7505, a microphone7506, and the like. The semiconductor device of one embodiment of thepresent invention can be used for a mobile memory, a CPU, or the likeincorporated in the housing 7501. Note that the display portion 7502 issmall- or medium-sized but can perform full high vision, 4 k, or 8 kdisplay because it has greatly high definition; therefore, asignificantly clear image can be obtained.

FIG. 53D illustrates a video camera, which includes a first housing7701, a second housing 7702, a display portion 7703, operation keys7704, a lens 7705, a joint 7706, and the like. The operation keys 7704and the lens 7705 are provided for the first housing 7701, and thedisplay portion 7703 is provided for the second housing 7702. The firsthousing 7701 and the second housing 7702 are connected to each otherwith the joint 7706, and the angle between the first housing 7701 andthe second housing 7702 can be changed with the joint 7706. Imagesdisplayed on the display portion 7703 may be switched in accordance withthe angle at the joint 7706 between the first housing 7701 and thesecond housing 7702. The imaging device in one embodiment of the presentinvention can be provided in a focus position of the lens 7705. Thesemiconductor device of one embodiment of the present invention can beused for an integrated circuit, a CPU, or the like incorporated in thefirst housing 7701.

FIG. 53E illustrates a digital signage including a display portion 7922provided on a utility pole 7921. The display device of one embodiment ofthe present invention can be used for a control circuit of the displayportion 7922.

FIG. 54A illustrates a notebook personal computer, which includes ahousing 8121, a display portion 8122, a keyboard 8123, a pointing device8124, and the like. The semiconductor device of one embodiment of thepresent invention can be used for a CPU, a memory, or the likeincorporated in the housing 8121. Note that the display portion 8122 issmall- or medium-sized but can perform 8 k display because it hasgreatly high definition; therefore, a significantly clear image can beobtained.

FIG. 54B is an external view of an automobile 9700. FIG. 54C illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The semiconductor device of one embodiment of the present invention canbe used in a display portion and a control integrated circuit of theautomobile 9700. For example, the semiconductor device of one embodimentof the present invention can be used in display portions 9710 to 9715illustrated in FIG. 54C.

The display portion 9710 and the display portion 9711 are displaydevices or input/output devices provided in an automobile windshield.The display device or input/output device of one embodiment of thepresent invention can be a see-through display device or input/outputdevice, through which the opposite side can be seen, by using alight-transmitting conductive material for its electrodes. Such asee-through display device or input/output device does not hinderdriver's vision during the driving of the automobile 9700. Therefore,the display device or input/output device of one embodiment of thepresent invention can be provided in the windshield of the automobile9700. Note that in the case where a transistor or the like for drivingthe display device or input/output device is provided in the displaydevice or input/output device, a transistor having light-transmittingproperties, such as an organic transistor using an organic semiconductormaterial or a transistor using an oxide semiconductor, is preferablyused.

The display portion 9712 is a display device provided on a pillarportion. For example, an image taken by an imaging unit provided in thecar body is displayed on the display portion 9712, whereby the viewhindered by the pillar portion can be compensated. The display portion9713 is a display device provided on the dashboard. For example, animage taken by an imaging unit provided in the car body is displayed onthe display portion 9713, whereby the view hindered by the dashboard canbe compensated. That is, by displaying an image taken by an imaging unitprovided on the outside of the automobile, blind areas can be eliminatedand safety can be increased. Displaying an image to compensate for thearea which a driver cannot see, makes it possible for the driver toconfirm safety easily and comfortably.

FIG. 54D illustrates the inside of a car in which a bench seat is usedas a driver seat and a front passenger seat. A display portion 9721 is adisplay device or input/output device provided in a door portion. Forexample, an image taken by an imaging unit provided in the car body isdisplayed on the display portion 9721, whereby the view hindered by thedoor can be compensated. A display portion 9722 is a display deviceprovided in a steering wheel. A display portion 9723 is a display deviceprovided in the middle of a seating face of the bench seat. Note thatthe display device can be used as a seat heater by providing the displaydevice on the seating face or backrest and by using heat generation ofthe display device as a heat source.

The display portion 9714, the display portion 9715, and the displayportion 9722 can display a variety of kinds of information such asnavigation data, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be changed freely bya user as appropriate. The information listed above can also bedisplayed on the display portions 9710 to 9713, 9721, and 9723. Thedisplay portions 9710 to 9715 and 9721 to 9723 can also be used aslighting devices. The display portions 9710 to 9715 and 9721 to 9723 canalso be used as heating devices.

FIG. 55A illustrates an external view of a camera 8000. The camera 8000includes a housing 8001, a display portion 8002, an operation button8003, a shutter button 8004, a connection portion 8005, and the like. Alens 8006 can be put on the camera 8000.

The connection portion 8005 includes an electrode to connect with afinder 8100, which is described below, a stroboscope, or the like.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in ahousing.

Images can be taken at the touch of the shutter button 8004. Inaddition, images can be taken at the touch of the display portion 8002which serves as a touch panel.

The display device or input/output device of one embodiment of thepresent invention can be used in the display portion 8002.

FIG. 55B shows the camera 8000 with the finder 8100 connected.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a connection portion for the camera 8000 andthe connection portion 8005, and the finder 8100 can be connected to thecamera 8000. The connection portion includes an electrode, and an imageor the like received from the camera 8000 through the electrode can bedisplayed on the display portion 8102.

The button 8103 has a function of a power button, and the displayportion 8102 can be turned on and off with the button 8103.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit and an image sensor included in thehousing 8101.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIGS. 55A and 55B, the housing 8001 of the camera8000 may include a finder having the display device or input/outputdevice of one embodiment of the present invention.

FIG. 55C illustrates an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. In addition, the movement of the eyeball and theeyelid of a user can be captured by a camera in the main body 8203 andthen coordinates of the points the user looks at can be calculated usingthe captured data to utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes to be incontact with the user. The main body 8203 may be configured to sensecurrent flowing through the electrodes with the movement of the user'seyeball to recognize the direction of his or her eyes. The main body8203 may be configured to sense current flowing through the electrodesto monitor the user's pulse. The mounting portion 8201 may includesensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit included in the main body 8203.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 14

In this embodiment, application examples of an RF tag using thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIGS. 56A to 56F.

<Application Examples of RF Tag>

The RF tag is widely used and can be provided for, for example, productssuch as bills, coins, securities, bearer bonds, documents (e.g.,driver's licenses or resident's cards, see FIG. 56A), vehicles (e.g.,bicycles, see FIG. 56B), packaging containers (e.g., wrapping paper orbottles, see FIG. 56C), recording media (e.g., DVD or video tapes, seeFIG. 56D), personal belongings (e.g., bags or glasses), foods, plants,animals, human bodies, clothing, household goods, medical supplies suchas medicine and chemicals, and electronic devices (e.g., liquid crystaldisplay devices, EL display devices, television sets, or cellularphones), or tags on products (see FIGS. 56E and 56F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 of one embodiment of the present invention can be reduced insize, thickness, and weight, it can be fixed to a product withoutspoiling the design of the product. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have anidentification function by being provided with the RF tag 4000 of oneembodiment of the present invention, and the identification function canbe utilized to prevent counterfeiting. Moreover, the efficiency of asystem such as an inspection system can be improved by providing the RFtag of one embodiment of the present invention for packaging containers,recording media, personal belongings, foods, clothing, household goods,electronic devices, or the like. Vehicles can also have higher securityagainst theft or the like by being provided with the RF tag of oneembodiment of the present invention.

As described above, by using the RF tag including the semiconductordevice of one embodiment of the present invention for each applicationdescribed in this embodiment, power for operation such as writing orreading of data can be reduced, which results in an increase in themaximum communication distance. Moreover, data can be held for anextremely long period even in the state where power is not supplied;thus, the RF tag can be preferably used for application in which data isnot frequently written or read.

This embodiment can be combined as appropriate with any of the otherembodiments in this specification.

EXPLANATION OF REFERENCE

200: imaging device, 201: switch, 202: switch, 203: switch, 210: pixelportion, 211: pixel, 212: subpixel, 212B: subpixel, 212G: subpixel,212R: subpixel, 220: photoelectric conversion element, 230: pixelcircuit, 231: wiring, 247: wiring, 248: wiring, 249: wiring, 250:wiring, 253: wiring, 254: filter, 254B: filter, 254G: filter, 254R:filter, 255: lens, 256: light, 257: wiring, 260: peripheral circuit,270: peripheral circuit, 280: peripheral circuit, 290: peripheralcircuit, 291: light source, 300: silicon substrate, 301: insulator, 303:insulator, 305: layer, 310: conductor, 320: layer, 330: transistor, 331:layer, 340: layer, 351: transistor, 352: transistor, 353: transistor,354: transistor, 360: photodiode, 361: anode, 362: cathode, 363:low-resistance region, 365: photodiode, 366: semiconductor layer, 367:semiconductor layer, 368: semiconductor layer, 370: plug, 371: wiring,372: wiring, 373: wiring, 374: wiring, 380: insulator, 381: insulator,400: substrate, 401: insulator, 402: insulator, 404: conductor, 404_1:conductor, 404 w: gate line width, 406 a: insulator, 406 b:semiconductor, 406 c: insulator, 406 c_1: insulator, 407: region, 408:insulator, 410: insulator, 410_1: insulator, 410 a: insulator, 411:resist mask, 412: insulator, 412_1: insulator, 413: organic substance,414: mixed region, 415: conductor, 416 a 1: conductor, 416 a 2:conductor, 416 w 1: opening width, 416 w 2: opening width, 428:insulator, 429: conductor, 430: conductor, 431: conductor, 432:conductor, 437: conductor, 438: conductor, 440: conductor, 442:conductor, 444: conductor, 446: taper angle, 447: taper angle, 450:semiconductor substrate, 454: conductor, 460: region, 462: insulator,464: insulator, 465: insulator, 466: insulator, 467: insulator, 468:insulator, 469: insulator, 470: insulator, 472: insulator, 474 a:region, 474 b: region, 475: insulator, 476 a: conductor, 476 b:conductor, 476 c: conductor, 477 a: conductor, 477 b: conductor, 477 c:conductor, 478 a: conductor, 478 b: conductor, 478 c: conductor, 479 a:conductor, 479 b: conductor, 479 c: conductor, 480 a: conductor, 480 b:conductor, 480 c: conductor, 483 a: conductor, 483 b: conductor, 483 c:conductor, 483 d: conductor, 483 e: conductor, 483 f: conductor, 484 a:conductor, 484 b: conductor, 484 c: conductor, 484 d: conductor, 485 a:conductor, 485 b: conductor, 485 c: conductor, 485 d: conductor, 487 a:conductor, 487 b: conductor, 487 c: conductor, 488 a: conductor, 488 b:conductor, 488 c: conductor, 489 a: conductor, 489 b: conductor, 490 a:conductor, 490 b: conductor, 491 a: conductor, 491 b: conductor, 491 c:conductor, 492 a: conductor, 492 b: conductor, 492 c: conductor, 494:conductor, 496: conductor, 498: insulator, 550: interposer, 551: chip,552: terminal, 553: mold resin, 600: panel, 601: printed wiring board,602: package, 603: FPC, 604: battery, 700: substrate, 704 a: conductor,704 b: conductor, 706 a: insulator, 706 b: semiconductor, 706 c:insulator, 706 d: insulator, 706 e: semiconductor, 706 f: insulator,710: insulator, 712 a: insulator, 714 a: conductor, 714 b: conductor,716 a 1: conductor, 716 a 2: conductor, 716 a 3: conductor, 716 a 4:conductor, 718 b: insulator, 719: light-emitting element, 720:insulator, 721: insulator, 728: insulator, 731: terminal, 732: FPC, 733a: wiring, 734: sealant, 735: driver circuit, 736: driver circuit, 737:pixel, 741: transistor, 742: capacitor, 743: switching element, 744:signal line, 750: substrate, 751: transistor, 752: capacitor, 753:liquid crystal element, 754: scan line, 755: signal line, 781:conductor, 782: light-emitting layer, 783: conductor, 784: partitionwall, 791: conductor, 792: insulator, 793: liquid crystal layer, 794:insulator, 795: spacer, 796: conductor, 797: substrate, 800: RF tag,801: communication device, 802: antenna, 803: radio signal, 804:antenna, 805: rectifier circuit, 806: constant voltage circuit, 807:demodulation circuit, 808: modulation circuit, 809: logic circuit, 810:memory circuit, 811: ROM, 1189: ROM interface, 1190: substrate, 1191:ALU, 1192: ALU controller, 1193: instruction decoder, 1194: interruptcontroller, 1195: timing controller, 1196: register, 1197: registercontroller, 1198: bus interface, 1199: ROM, 1200: memory element, 1201:circuit, 1202: circuit, 1203: switch, 1204: switch, 1206: logic element,1207: capacitor, 1208: capacitor, 1209: transistor, 1210: transistor,1213: transistor, 1214: transistor, 1220: circuit, 2100: transistor,2200: transistor, 2201: insulator, 2202: wiring, 2203: plug, 2204:insulator, 2205: wiring, 2207: insulator, 2210: intermediate layer,2211: semiconductor substrate, 2212: insulator, 2213: gate electrode,2214: gate insulator, 2215: source and drain regions, 3001: wiring,3002: wiring, 3003: wiring, 3004: wiring, 3005: wiring, 3200:transistor, 3300: transistor, 3400: capacitor, 4000: RF tag, 5100:pellet, 5120: substrate, 5161: region, 6000: display module, 6001: uppercover, 6002: lower cover, 6003: FPC, 6004: touch panel, 6005: FPC, 6006:display panel, 6007: backlight unit, 6008: light source, 6009: frame,6010: printed board, 6011: battery, 7101: housing, 7102: housing, 7103:display portion, 7104: display portion, 7105: microphone, 7106: speaker,7107: operation key, 7108: stylus, 7302: housing, 7304: display portion,7311: operation button, 7312: operation button, 7313: connectionterminal, 7321: band, 7322: clasp, 7501: housing, 7502: display portion,7503: operation button, 7504: external connection port, 7505: speaker,7506: microphone, 7701: housing, 7702: housing, 7703: display portion,7704: operation key, 7705: lens, 7706: joint, 7901: utility pole, 7902:display portion, 8000: camera, 8001: housing, 8002: display portion,8003: operation button, 8004: shutter button, 8005: connection portion,8006: lens, 8100: finder, 8101: housing, 8102: display portion, 8103:button, 8121: housing, 8122: display portion, 8123: keyboard, 8124:pointing device, 8200: head-mounted display, 8201: mounting portion,8202: lens, 8203: main body, 8204: display portion, 8205: cable, 8206:battery, 9700: automobile, 9701: car body, 9702: wheels, 9703:dashboard, 9704: lights, 9710: display portion, 9711: display portion,9712: display portion, 9713: display portion, 9714: display portion,9715: display portion, 9721: display portion, 9722: display portion,9723: display portion

This application is based on Japanese Patent Application serial no.2015-041204 filed with Japan Patent Office on Mar. 3, 2015, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first transistor comprising achannel formation region in a semiconductor substrate; a first insulatorover the semiconductor substrate; a first conductor functioning as afirst gate electrode of the first transistor over the first insulator; asecond transistor over the first transistor comprising an oxidesemiconductor in a channel formation region; and a capacitor over thesecond transistor, wherein one of a source and a drain of the firsttransistor is electrically connected to one of a source and a drain ofthe second transistor, wherein the first gate electrode of the firsttransistor is electrically connected to the other of the source and thedrain of the second transistor and one electrode of the capacitor,wherein a second insulator is provided over the channel formation regionof the second transistor, and wherein a second conductor functioning asa first gate electrode of the second transistor is provided over thesecond insulator.
 2. A semiconductor device comprising: a firsttransistor comprising silicon in a channel formation region; a firstinsulator over the channel formation region of the first transistor; afirst conductor functioning as a first gate electrode of the firsttransistor over the first insulator; a second transistor overlappingwith the first transistor comprising an oxide semiconductor in a channelformation region; and a capacitor overlapping with the secondtransistor, wherein one of a source and a drain of the first transistoris electrically connected to one of a source and a drain of the secondtransistor via a first plurality of conductors provided in a firstplurality of insulators, wherein the first gate electrode of the firsttransistor is electrically connected to the other of the source and thedrain of the second transistor and one electrode of the capacitor via asecond plurality of conductors provided in the first plurality ofinsulators, wherein the other of the source and the drain of the firsttransistor is electrically connected to a conductor provided over thecapacitor, wherein a second insulator is provided over the channelformation region of the second transistor, wherein a second conductorfunctioning as a first gate electrode of the second transistor isprovided over the second insulator.
 3. A semiconductor devicecomprising: a first transistor comprising silicon in a channel formationregion; a first insulator over the channel formation region of the firsttransistor; a first conductor functioning as a first gate electrode ofthe first transistor over the first insulator; a second transistoroverlapping with the first transistor comprising an oxide semiconductorin a channel formation region; and a capacitor overlapping with thesecond transistor, wherein a second insulator is provided over thechannel formation region of the second transistor, wherein a secondconductor functioning as a first gate electrode of the second transistoris provided over the second insulator, wherein one of a source and adrain of the first transistor is electrically connected to one of asource and a drain of the second transistor via a third conductorprovided over the second conductor, wherein the first gate electrode ofthe first transistor is electrically connected to the other of thesource and the drain of the second transistor and one electrode of thecapacitor via a fourth conductor provided over the second conductor,wherein the other of the source and the drain of the first transistor iselectrically connected to a fifth conductor provided over the otherelectrode of the capacitor, and wherein the other electrode of thecapacitor overlaps at least one of the second conductor, the firstconductor, the channel formation region of the first transistor, and thechannel formation region of the second transistor.
 4. The semiconductordevice according to claim 1, wherein the oxide semiconductor comprisesat least two of indium, gallium, zinc, and tin, wherein the channelformation region of the second transistor is provided on a protrusion ofa third insulator provided over the first conductor, and wherein sourceand drain regions of the first transistor overlap with source and drainregions of the second transistor.
 5. The semiconductor device accordingto claim 2, wherein the oxide semiconductor comprises at least two ofindium, gallium, zinc, and tin, wherein the channel formation region ofthe second transistor is provided on a protrusion of a third insulatorprovided over the first conductor, and wherein source and drain regionsof the first transistor overlap with source and drain regions of thesecond transistor.
 6. The semiconductor device according to claim 3,wherein the oxide semiconductor comprises at least two of indium,gallium, zinc, and tin, wherein the channel formation region of thesecond transistor is provided on a protrusion of a third insulatorprovided over the first conductor, and wherein source and drain regionsof the first transistor overlap with source and drain regions of thesecond transistor.
 7. The semiconductor device according to claim 1,wherein a portion of a fourth insulator is embedded in the oxidesemiconductor, wherein a fifth insulator is in contact with a sidesurface of the second insulator, and wherein a sixth insulator is incontact with a side surface of the fifth insulator.
 8. The semiconductordevice according to claim 1, wherein the one electrode of the capacitorsurrounds the other electrode of the capacitor.
 9. The semiconductordevice according to claim 2, wherein the one electrode of the capacitorsurrounds the other electrode of the capacitor.
 10. The semiconductordevice according to claim 3, wherein the one electrode of the capacitorsurrounds the other electrode of the capacitor.